Adds custom 404 page

[melissawm]

Closes napari/napari-sphinx-theme#125

This is a proposal to

1. Add a custom 404 page to the website
2. Remove all extra files that are now unnecessary to the website. The files at the root of the repo (such as usage.html and others like tutorials/*) were acting as redirects to stable, and with a 404 page I think we can remove them. The .doctrees and _sources/ folders are not necessary to the website, so we can remove them here and also make sure they are not copied over from commits to napari/napari and napari/docs (in a follow up PR)