DOC: Adding redirects to API moved pages
- [X] closes #24451
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
I suppose we don't care too much about adding unneeded redirects? (or at least now) 
Because eg all the dtypes (except for Categorical) and all the arrays were not yet exposed in 0.23.4 It's easy to just add the ones for the pages in 0.23.4. But I can't do it right now, can do it later tonight or tomorrow. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24909?src=pr&el=h1) Report
> Merging [#24909](https://codecov.io/gh/pandas-dev/pandas/pull/24909?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/fa12b9ecbea36a504249e74579ee2124f98bcc3f?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24909/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24909?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24909   +/-   ##
=======================================
  Coverage   92.38%   92.38%           
=======================================
  Files         166      166           
  Lines       52402    52402           
=======================================
  Hits        48414    48414           
  Misses       3988     3988
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.81% <ø> (ø)` | :arrow_up: |
| #single | `42.89% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24909?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24909?src=pr&el=footer). Last update [fa12b9e...03292ac](https://codecov.io/gh/pandas-dev/pandas/pull/24909?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24909?src=pr&el=h1) Report
> Merging [#24909](https://codecov.io/gh/pandas-dev/pandas/pull/24909?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/013ae3daf2b6793478f58e9e88d6d36c308f69b7?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24909/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24909?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24909   +/-   ##
=======================================
  Coverage   42.89%   42.89%           
=======================================
  Files         166      166           
  Lines       52389    52389           
=======================================
  Hits        22473    22473           
  Misses      29916    29916
```

| Flag | Coverage Δ | |
|---|---|---|
| #single | `42.89% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24909?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24909?src=pr&el=footer). Last update [013ae3d...f85fc9a](https://codecov.io/gh/pandas-dev/pandas/pull/24909?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @datapythonista added a clean-up commit, and then also a commit renaming to my last proposal in the issue (`reference/api/`). OK with that? 
(it's in a separate commit, so easy to revert) @TomAugspurger can you do a quick sanity check of the clean-up commit (https://github.com/pandas-dev/pandas/pull/24909/commits/b9a300e87adce980b383a84892e3faea918b80bf)? 
I only removed the obvious ones (top level dtypes, arrays, and a bunch of png/py files, didn't check in detail for other possible methods that might have been added in 0.24.0. In any case this should be the big bulk).

Doing a local doc build to check. Looks good to me, I like using `reference/api`. Couldn't check the list of redirects in detail, I have a work emergency I need to take care of this morning, but hopefully I can take a closer look in couple of hours.  `ssource/reference/index.rst` still has references to `generated/`

```
.. toctree::
   :hidden:

   generated/pandas.DataFrame.blocks
   generated/pandas.DataFrame.as_matrix
   generated/pandas.DataFrame.ix
 ...
``` Good catch, I only searched for `:toctree: generated` (and we actually still had a faulty one like that in the user guide, so that made a `user_guide/generated` TODO: check if single-page build works. that has a reference to generated

```
{% elif single_doc %}
.. autosummary::
    :toctree: api/generated/

    {{ single_doc }}
```

edit: looks like it'll probably work, since the file is written to `/api.generated` still. Not sure if that should be updated too. @TomAugspurger I just included that in a fixup
 I now checked all occurences of 'generated' in the doc source, and the remaining should be OK OK, did a local build, and also tried a single docstring page, and seems to be good. Let's merge so we can still do a quick check with the travis built docs?