Extension notfound.extension is not needed

[dviererbe]

I know that we discussed in Riga, but I still think that we do not need the notfound.extension. ReadTheDocs will automatically serve the content of /404.html or 404/index.html if a page could not be found.

See: https://docs.readthedocs.io/en/stable/reference/404-not-found.html

[pmatulis]

Deferring to @ru-fu on this one.

[ru-fu]

Yes, I'll test that in connection with #138. I still think it's needed to get the style sheets to display correctly.

[pmatulis]

Resolved by #180