DOC: add pandas-gbq and related db-dtypes packages to ecosystem
As of pandas 2.2.0, pandas-gbq functionality is fully separated, so I think now would be a good time to introduce it as part of the "ecosystem".

db-dtypes was built for use from pandas-gbq but should be applicable more generally.

- [N/A] closes #xxxx (Replace xxxx with the GitHub issue number)
- [N/A] [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
- [N/A] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [N/A] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [N/A] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Thanks @tswast 