Extension for Ansible redirects

[oraNod]

Fixes #2147 and supercedes #2266

DRAFT: Still need to add some docs and other improvements but would like to get early feedback.

This PR extends the Sphinx reredirects extension to load redirects for Ansible documentation from a yaml file. The purpose is to replace the httpd mod_rewrite rules in https://github.com/ansible/docsite/blob/main/ansible/11/.htaccess and https://github.com/ansible/docsite/blob/main/.htaccess in a way that is compatible with ReadTheDocs.

The extension is enabled when ENABLE_REDIRECTS is true. You can achieve this with export ENABLE_REDIRECTS=1. In this way the extension does not run for core doc builds or when users build docs locally. We want to avoid this because the extension generates a large number of HTML files in the build directory, which can be really noisy.

If this PR is accepted and merged, there are two follow on actions:

• Create a robots.txt file to stop crawlers from indexing the generated HTML files. We can put the robots.txt file in the ansible/docsite repo and copy it across as part of the ReadTheDocs build.
• Remove all the existing stub files from this repository. For example, all the files under the docs/docsite/rst/user_guide folder.