DOC: add NaN example to DataFrame.equals

[ingo-stallknecht]

• closes #xxxx (Replace xxxx with the GitHub issue number)
• 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.
• If I used AI to develop this pull request, I prompted it to follow AGENTS.md.

This PR updates the DataFrame.equals docstring to include a short example
showing how NaN values are handled.

• Adds an example where NaN values are in the same locations, which compares True.
• Adds an example where the position of NaN differs, which compares False.

Documentation-only change, no behavior modifications.

[mroeschke]

Thanks @ingo-stallknecht