DOC: Add docstrings for fixtures defined from pandas.tests.io.conftest
Add docstrings for fixtures defined from pandas.tests.io.conftest

- [x] xref https://github.com/pandas-dev/pandas/issues/19159
- [x] [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.

@datapythonista would you please give this PR a review and share any feedback?  Is there any validation for it we are trying to enforce? I personally don't see much value here otherwise. It'd be nice to have proper documentation for fixtures for sure, but that would take understanding well them and providing useful information on what they are exactly and when they can be used. This PR seems to simply repeat the fixture names in a more verbose way. I'd prefer to not have them rather than this. Agreed with @datapythonista on the value of this. If you're interested, continuing to enforce numpydoc validations would be worthwhile: https://numpydoc.readthedocs.io/en/latest/validation.html#built-in-validation-checks @datapythonista @mroeschke 

Thank you gentlemen, I appreciate the feedback and guidance! 

One way I do think this adds value is when running `pytest --fixtures pandas` the output before these changes is: 

<img width="1133" alt="before" src="https://github.com/pandas-dev/pandas/assets/35613487/32d1ab5c-70f2-4493-b9dc-23a5f22285af">

and after adding the docstrings: 

<img width="1320" alt="after" src="https://github.com/pandas-dev/pandas/assets/35613487/01d681ba-f371-4df7-ac36-be0937663bef">


Considering this, is this meaningful to add? If not, I'm happy to revert these changes and move on to working on continuing to enforce numpydoc validations as Matthew suggested. 

 > Considering this, is this meaningful to add?

Just to make my last comment clearer - I would still be OK with documenting these pytest fixtures, but I don't think there's a ton of value (impact) in doing so. @mroeschke I've opened https://github.com/pandas-dev/pandas/issues/57111 to continue work on enforcing numpydoc validations: https://numpydoc.readthedocs.io/en/latest/validation.html#built-in-validation-checks
