feat(docs): friendly 404 page

[joshtrichards]

☑️ Resolves

Add a proper 404 not found page.

How:

• Uses the sphinx-notfound-page extension (which Read the Docs maintains).
• The extension automatically creates a 404.html page for our docs that always matches our theme.

🖼️ Screenshots

Previous (current) behavior:

New:

[rakekniven]

@joshtrichards Does not work like expected ... any other steps to do?

[joshtrichards]

Not offhand, but my wild guess is that it has to do with being shared across the three separate manuals. Looks like some URLs are also not loading even when loading the generated HTML directly at /[...]/404.html. I'll stick in on my list for follow-up when I can.