Doc: add ExtensionArray.map to reference/extensions.rst
xref #52247 & #52263.
This is the error that also failed last time

```
2023-03-29T20:13:24.9068667Z /home/runner/work/pandas/pandas/doc/source/reference/api/pandas.api.extensions.ExtensionArray.map.rst: WARNING: document isn't included in any toctree
``` Yeah, I’m currently at a loss why this fails. I’l look at this again to see if I can figure it out. /preview No preview found for PR #52280. Did the docs build complete? Still as a loss why this fails. Error message still the same (`pandas/doc/source/reference/api/pandas.api.extensions.ExtensionArray.map.rst: WARNING: document isn't included in any toctree`).

Any suggestion welcome :-) This pull request is stale because it has been open for thirty days with no activity. Please [update](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#updating-your-pull-request) and respond to this comment if you're still interested in working on this. Looks like this PR is stale and the check is still failing so closing for now. Feel free to reopen when you want to get back to this PR