Update groupby().first() documentation to clarify behavior with missing data (#27578)
This PR enhances the docstring for `GroupBy.first()` to clarify:
- It returns the first *non-null* value per column
- It differs from `.nth(0)` and `.head(1)` in how it treats missing values
- Includes comparative examples for better understanding

Fixes part of issue #27578

Ready for review.
Thanks for the pull request, but it appears to have gone stale. If interested in continuing, please merge in the main branch, address any review comments and/or failing tests, and we can reopen.