add 404 page to docs

[maxpietsch]

implements a 404 page, replacing the default Maze page

• uses the sphinx-notfound-page plugin
• had to install it explicitly via pip in github workflow, updated the README and added a requirements.txt file in docs/
• added the dependency to a new readthedocs yml config file

With this plugin, loading a nonexistent URL still emits a 404 signal.

One issue is that on the 404.html page, if one selects a different version of the docs from the menu, one is redirected to a 404.html page relative to that version (which does not exist for older versions) instead of reusing and following the actual link.
For instance: https://mrtrix.readthedocs.io/en/docs_404/does_not_exist --> https://mrtrix.readthedocs.io/en/latest/404.html instead of
https://mrtrix.readthedocs.io/en/latest/does_not_exist

[maxpietsch]

cherry-picked 4d46989 aad948b and 60acb3b into doctest