DOC: Fix doc for first_valid_index

[jfadia]

• closes DOC:  #55187
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations 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.

[mroeschke]

Could you just remove this Notes section and demonstrate these points in the Examples section below?

[jfadia]

Done. One thing I was unsure of was how to handle returning None in the docs:

• I ran it through the docstring validation script python scripts/validate_docstrings.py pandas.core.generic.NDFrame.first_valid_index
• It was throwing errors about the output not matching the expected output because I had originally specified:

        >>> s = pd.Series()
        >>> s.first_valid_index()
        None
        >>> s.last_valid_index()
        None

I considered printing the outputs so that the outputs would match but didn't see print statements used in that way in other parts of the code so I did not keep them. Let me know if printing is the way to go here and I can make the change :)

[jfadia]

To clarify, option 1:

>>> s = pd.Series()
>>> s.first_valid_index()
>>> s.last_valid_index()

Option 2:

>>> s = pd.Series()
>>> print(s.first_valid_index())
None
>>> print(s.last_valid_index())
None

[mroeschke]

Option 2 would be fine to make it explicit in the docs

[jfadia]

Added the print statements

[mroeschke]

Thanks @jfadia