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