BUG/DOC: Update dtype_backend documentation and API

[jacgoldsm]

• closes ENH:  #55647
• Corrects https://pandas.pydata.org/docs/reference/api/pandas.read_csv.html incorrectly stating that the default dtype_backend was numpy_nullable
• 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.

[jacgoldsm]

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

[github-actions]

This pull request is stale because it has been open for thirty days with no activity. Please update and respond to this comment if you're still interested in working on this.

[mroeschke]

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.