DOC: contributing.rst sphinx link leads to 400 bad request

[tonywu1999]

When reading through the contributing guidelines (https://dev.pandas.io/docs/development/contributing.html), there are links to the Sphinx documentation and an introduction to reST.

Sphinx documentation: http://sphinx.pocoo.org/
Introduction to reST: http://sphinx.pocoo.org/rest.html

However, these links are not working and are leading to 400 bad requests.

I propose we replace these links with the following links:
Sphinx documentation: http://www.sphinx-doc.org/en/master/
Introduction to reST: https://www.sphinx-doc.org/en/master/usage/restructuredtext/index.html

I think this will be an easy fix, editing the links in the contributing.rst file in the documentation folder.

[MarcoGorelli]

Thanks for the report!

I agree, but perhaps https://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html is more suited as the link for "Introduction to reST"?

[tonywu1999]

take

[simonjayhawkins]

xref #31011