add Sphinx notfound extension

[acharraggi]

Add extension to create a custom 404 not found page.
Recommended by RTD here: https://docs.readthedocs.com/platform/stable/reference/404-not-found.html

This PR is not ready to review.
Updated both requirements.txt and conf.py.

[acharraggi]

Here's the default 404 page generated by the notfound extension. Not too exciting.
At least it has FTC Docs styling and headers, footers.

FYI - Currently FTC Docs doesn't generate a 404 page and the browser does this with a HTML 404 response.

[acharraggi]

The Read the Docs build is here: https://first-tech-challenge-ftcdocs--390.com.readthedocs.build/en/390/index.html

Here's the current message. I mention using the sidebar links and search box as well as the FTC Docs Archive for Tensorflow stuff. I think that might be enough. I don't think we need to too many general links as they become one more thing to try and keep up to date. People can use the sidebar menu to find stuff.