DOC: Add SeriesGroupBy and DataFrameGroupBy reference pages
- [x] closes #48399  (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
- [x] 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.

I grepped py / rst files for any references to `GroupBy.`:

<details>
<summary>grep results</summary>

    grep -rl --include \*.rst --include \*.py "[^se]GroupBy\."

pandas/core/sample.py
pandas/core/resample.py
pandas/core/apply.py
pandas/core/groupby/grouper.py
pandas/core/groupby/groupby.py
pandas/core/groupby/indexing.py
pandas/tests/plotting/test_groupby.py
pandas/tests/groupby/test_indexing.py
pandas/tests/groupby/test_pipe.py
doc/source/whatsnew/v1.0.1.rst
doc/source/whatsnew/v0.18.0.rst
doc/source/whatsnew/v0.25.2.rst
doc/source/whatsnew/v1.0.4.rst
doc/source/whatsnew/v0.21.0.rst
doc/source/whatsnew/v0.14.0.rst
doc/source/whatsnew/v1.4.0.rst
doc/source/whatsnew/v0.20.0.rst
doc/source/whatsnew/v1.3.3.rst
doc/source/whatsnew/v1.5.0.rst
doc/source/whatsnew/v0.24.2.rst
doc/source/whatsnew/v1.1.5.rst
doc/source/whatsnew/v1.2.1.rst
doc/source/whatsnew/v0.16.1.rst
doc/source/whatsnew/v1.1.4.rst
doc/source/whatsnew/v0.16.2.rst
doc/source/whatsnew/v0.20.2.rst
doc/source/whatsnew/v0.23.1.rst
doc/source/whatsnew/v1.3.0.rst
doc/source/whatsnew/v1.0.0.rst
doc/source/whatsnew/v0.25.0.rst
doc/source/whatsnew/v0.24.0.rst
doc/source/whatsnew/v1.1.0.rst
doc/source/whatsnew/v0.25.1.rst
doc/source/whatsnew/v1.3.5.rst
doc/source/whatsnew/v1.3.4.rst
doc/source/whatsnew/v1.6.0.rst
doc/source/whatsnew/v0.23.0.rst
doc/source/whatsnew/v1.2.0.rst
doc/source/whatsnew/v0.14.1.rst
doc/source/whatsnew/v0.15.0.rst
doc/source/whatsnew/v0.18.1.rst
doc/source/whatsnew/v1.0.2.rst
</details>

Looking through the py files, all remaining references are internal. This only leaves what to do with the whatsnew files. I could  

 - replace all occurrences;
 - replace all occurrences in 1.x files;
 - replace all occurrences in 1.4/1.5/1.6 files;
 - leave any past whatsnews alone

Any thoughts @mroeschke?
If there's not too many occurrences, it would be great to replace all the occurrences in the whatsnew files (I've wanted to do this for other references too but low priority). Otherwise, might be good just to do the recent ones (1.4/1.5/1.6). Nice! Thanks @rhshadrach 