Add information about code coverage to docs
Hello! This change adds code coverage documentation and also includes a link to the Codecov dashboard for Pandas within the contributing docs. Some of this information is otherwise implicit and I thought could be helpful for developers to understand or have an easy link handy.

Thank you for any thoughts or feedback you may have! And cheers on the great work with Pandas more broadly.

- [x] closes #60010 
- [ ] [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.


Thanks @d33bs  Thanks as well @mroeschke !