DOC: Fix doc redirects
- [ ] closes #xxxx (Replace xxxx with the GitHub issue number)
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Summary:

 - Removed any redirects for methods that have been removed/deprecated
 - Added/updated redirects for GroupBy -> DataFrameGroupby
 - Added redirects for EWM -> ExponentialMovingWindow
 - Added/updated redirects for alias pages we used to generate but the alias was removed/deprecated (e.g. backfill -> bfill, applymap -> map) or just the doc page was removed (e.g. notnull -> notna)

There are many old API pages that no longer exist but are still part of the API (or at least, I couldn't find deprecation/removal notes). These are in the `missing_methods` list in the script below. I'm also currently excluding `pandas.tseries`; there are too many potential issues to handle here.

<details>
<summary>Testing script</summary>

```
from pathlib import Path
import os

# Download each of the versions of docs listed below as zip files, unzip into a
# directory with subdirectories 1.1, 1.2, etc.
base_old = Path(f"/home/richard/Downloads/docs/")
base_pr = Path("/home/richard/dev/pandas/doc/build/html")

# We'll make sure redirects don't exist for any methods that have been removed / deprecated
removed_methods = [
    "pandas.DataFrame.append.html",
    "pandas.DataFrame.iteritems.html",
    "pandas.DataFrame.lookup.html",
    "pandas.DataFrame.mad.html",
    "pandas.DataFrame.slice_shift.html",
    "pandas.DataFrame.tshift.html",
    "pandas.DatetimeIndex.to_perioddelta.html",
    "pandas.DatetimeIndex.weekofyear.html",
    "pandas.Float64Index.html",
    "pandas.Index.asi8.html",
    "pandas.Index.get_value.html",
    "pandas.Index.holds_integer.html",
    "pandas.Index.is_all_dates.html",
    "pandas.Index.is_mixed.html",
    "pandas.Index.is_monotonic.html",
    "pandas.Index.is_type_compatible.html",
    "pandas.Index.set_value.html",
    "pandas.Index.to_native_types.html",
    "pandas.Int64Index.html",
    "pandas.MultiIndex.is_lexsorted.html",
    "pandas.Resampler.pad.html",
    "pandas.Series.append.html",
    "pandas.Series.is_monotonic.html",
    "pandas.Series.iteritems.html",
    "pandas.Series.mad.html",
    "pandas.Series.slice_shift.html",
    "pandas.Series.slice_shift.html",
    "pandas.Series.swapaxes.html",
    "pandas.Series.tshift.html",
    "pandas.Timedelta.delta.html",
    "pandas.Timedelta.is_populated.html",
    "pandas.Timestamp.freq.html",
    "pandas.Timestamp.freqstr.html",
    "pandas.UInt64Index.html",
    "pandas.api.types.is_categorical.html",
    "pandas.api.types.is_extension_type.html",
    "pandas.core.groupby.GroupBy.pad.html",
    "pandas.core.groupby.DataFrameGroupBy.mad.html",
    "pandas.core.groupby.DataFrameGroupBy.pad.html",
    "pandas.core.groupby.DataFrameGroupBy.tshift.html",
    "pandas.core.resample.Resampler.pad.html",
    "pandas.io.formats.style.Styler.hide_columns.html",
    "pandas.io.formats.style.Styler.hide_index.html",
    "pandas.io.formats.style.Styler.set_na_rep.html",
    "pandas.io.formats.style.Styler.set_precision.html",
]
# These methods no longer have doc pages but are still part of the API 
# (or at least, I couldn't find deprecation/removal notes)
missing_methods = [
    "pandas.DataFrame.flags.html",
    "pandas.DataFrame.isetitem.html",
    "pandas.DataFrame.to_numpy.html",
    "pandas.DatetimeIndex.week.html",
    "pandas.ExcelWriter.sheets.html",
    "pandas.ExcelFile.close.html",
    "pandas.ExcelWriter.supported_extensions.html",
    "pandas.ExcelWriter.write_cells.html",
    "pandas.ExcelWriter.handles.html",
    "pandas.ExcelWriter.datetime_format.html",
    "pandas.ExcelWriter.book.html",
    "pandas.ExcelWriter.close.html",
    "pandas.ExcelWriter.check_extension.html",
    "pandas.ExcelWriter.if_sheet_exists.html",
    "pandas.ExcelWriter.path.html",
    "pandas.ExcelWriter.date_format.html",
    "pandas.ExcelWriter.cur_sheet.html",
    "pandas.ExcelWriter.engine.html",
    "pandas.ExcelWriter.save.html",
    "pandas.Flags.allows_duplicate_labels.html",
    "pandas.Index.array.html",
    "pandas.Index.diff.html",
    "pandas.Index.format.html",
    "pandas.Index.groupby.html",
    "pandas.Index.round",
    "pandas.Index.infer_objects.html",
    "pandas.Index.isnull.html",
    "pandas.Index.nlevels.html",
    "pandas.Index.sort.html",
    "pandas.Index.sortlevel.html",
    "pandas.Index.to_flat_index.html",
    "pandas.Index.to_numpy.html",
    "pandas.NamedAgg.index.html",
    "pandas.NamedAgg.aggfunc.html",
    "pandas.NamedAgg.column.html",
    "pandas.NamedAgg.count.html",
    "pandas.PandasArray.html",
    "pandas.Series.axes.html",
    "pandas.Series.divmod.html",
    "pandas.Series.dt.week.html",
    "pandas.Series.dt.weekofyear.html",
    "pandas.Series.info.html",
    "pandas.Series.rdivmod.html",
    "pandas.Timedelta.freq.html",
    "pandas.Timedelta.resolution_string.html",
    "pandas.Timestamp.fromisocalendar.html",
    "pandas.Timestamp.fromisoformat.html",
    "pandas.api.extensions.ExtensionArray._ndarray_values.html",
    "pandas.api.extensions.ExtensionDtype.construct_array_type.html",
    "pandas.api.extensions.ExtensionDtype.construct_from_string.html",
    "pandas.api.extensions.ExtensionDtype.empty.html",
    "pandas.api.extensions.ExtensionDtype.is_dtype.html",
    "pandas.api.extensions.ExtensionDtype.kind.html",
    "pandas.api.extensions.ExtensionDtype.na_value.html",
    "pandas.api.extensions.ExtensionDtype.name.html",
    "pandas.api.extensions.ExtensionDtype.names.html",
    "pandas.api.extensions.ExtensionDtype.type.html",
    "pandas.api.indexers.BaseIndexer.get_window_bounds.html",
    "pandas.api.indexers.FixedForwardWindowIndexer.get_window_bounds.html",
    "pandas.api.indexers.VariableOffsetWindowIndexer.get_window_bounds.html",
    "pandas.core.window.Rolling.html",
    "pandas.errors.AccessorRegistrationWarning",
    "pandas.io.formats.style.Styler.render.html",
    "pandas.io.formats.style.Styler.template.html",
    "pandas.io.formats.style.Styler.where.html",
    "pandas.option_context.__call__",
    "pandas.window.Rolling.html",
]

def strip_str(haystack, needle, before):
    idx = haystack.find(needle)
    assert idx >= 0
    if before:
        result = haystack[idx + len(needle):]
    else:
        result = haystack[:idx]
    return result

for version in ["1.0", "1.1", "1.2", "1.3", "1.4", "1.5", "2.0", "2.1"]:
    base_old_api = base_old / version / "reference" / "api"
    base_pr_api = base_pr / "reference" / "api"

    print('Version:', version)
    for e in base_old_api.rglob("*.html"):
        relative_path = e.relative_to(base_old_api)
        
        if str(relative_path).startswith("pandas.tseries"):
            # Only looking at pandas.core redirects
            continue
        
        target_exists = os.path.exists(base_pr_api / relative_path)
        
        if any(e in str(relative_path) for e in removed_methods + missing_methods):
            if target_exists:
                print(f'  {relative_path}: Removed method has redirect')
            continue
            
        if not target_exists:
            print(f'  {relative_path}: Does not exist')
            continue

        contents = open(base_pr_api / relative_path).read()
        
        if "The page has been moved to" not in contents:
            # There is no redirect
            continue

        # Get the redirect path
        contents = strip_str(contents, "0;URL=", before=True)
        path = Path(strip_str(contents, '"/>\n', before=False))
        
        if not os.path.exists(base_pr_api / path):
            print(f'  {relative_path}: Redirect does not exist')
            continue

        contents = open(base_pr_api / path).read()
        if "The page has been moved to" in contents:
            print(f'  {relative_path}: Redirect is another redirect')
            
    print()
    print('-' * 20)
    print()
```

</details>
I believe only the redirects of the version you've chosen will be used. In other words, this PR has no impact on the docs when you look at 2.1 or earlier. thanks @rhshadrach 