DOC: Validate docstring directives
- [X] closes #27629
- [x] tests added / passed
- [x] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

For the moment, I have hardcoded the directives: 
- deprecated
- versionadded
- versionchanged

Nevertheless, I have noted there exist other directives used in pandas:
- code-block
- note
- plot
- warning

May I also include these? Any reviewer? ;) @gfyoung @datapythonista Thanks. All in GREEN (I had already fixed the docstrings #27621). Thanks @albertvillanova !