DOC: Add ignore_functions option to validate_docstrings.py
- [ ] 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
- [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.

Following @MarcoGorelli [comment](https://github.com/pandas-dev/pandas/issues/49968#issuecomment-1333467996), this PR adds an option to `validate_docstrings.py` to ignore functions. This allows to ignore the remaining `RT02` errors, described in https://github.com/pandas-dev/pandas/issues/49968,  in `ci/code_checks.sh`. This will also be useful for other documentation-related errors that required multiple fixes.

thanks for working on this! i'll take a look > I like that it allows to validate more things, and avoid making mistakes in docstrings already passing RT02, but at the same time this can start to become quite complex, if for each docstring error we need to keep a separate call and the list of docstrings not passing it.

True, but this should hopefully just be temporary - once the errors are fixed, then this separate call can be removed > True, but this should hopefully just be temporary - once the errors are fixed, then this separate call can be removed

I think we started fixing docstrings 5 years ago ;) But sure, seems fine to me if that's the plan/ Thanks for reviews! I did a small refactor to make the testing easier. I also rebased on `main`. > I think we started fixing docstrings 5 years ago ;)

EX02 is now enabled 🎉 

I've come to believe that the only way to get validation issues fixed is to get something running in CI as quickly as possible (with an exclusions list if necessary). So thanks again @Moisan for this script, looks like it ended up delivering!

Let's see if EX01 follows a similar fate 🤞  Excellent work. Thanks @MarcoGorelli for leading this!