BUG/DOC: Update dtype_backend documentation and API
- [x] closes #55647 
- [x]  Corrects [https://pandas.pydata.org/docs/reference/api/pandas.read_csv.html](documentation) incorrectly stating that the default `dtype_backend` was `numpy_nullable`
- [x] Adds `numpy` as an option for `dtype_backend`, equivalent to the current default (`lib.no_default`). Previously it was impossible to achieve the default behavior by passing in an explicit parameter, except by passing in `pandas._libs.no_default`. Passing the string "numpy" is now equivalent to passing `pandas._libs.no_default`.

It looks like tests specifically disallow users from passing `dtype_backend=numpy`. I'm curious why, and if it is intended that there be no easy way of explicitly passing in a parameter equivalent to the default. If that is the case, how should the default be labelled in the documentation? @lithomas1  This pull request is stale because it has been open for thirty days with no activity. Please [update](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#updating-your-pull-request) and respond to this comment if you're still interested in working on this. 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.