DOC: Move content in doc/README.rst to doc/source/contributing.rst

[datapythonista]

Following the discussion in #23097, we should check if there is content in doc/README.rst that is not in doc/source/contributing.rst. Then:

• Move that content to doc/source/contributing.rst if something is not there already.
• Replace all the content of doc/README.rst by simply a link to the contributing documentation.

In #23084 some flaws in doc/README.rst are pointed out. If those parts need to be used, we should fix them as suggested in the issue.

[KimiJL]

Hi, I would like to claim this issue if possible, thanks!

[datapythonista]

Sure, thanks @KimiJL, let us know if you need help.