This change allows documentation page paths to be redirected to other

pages. We follow what we do for our Review Board codebase docs, where we:

- Add a redirects.json file that contains the redirect info

- Point to that file in collect_file_patterns in our conf.py

- Rely on the beanbag_docutils.sphinx.ext.collect_files extension to

  collect the redirect info.
The redirect info will be filled out in other changes.

Used in other changes.