Basic 404 page.

Merged username-removed-25042 requested to merge pserwylo/fdroid-website:404 into master

Happy to take suggestions on the wording for this page, but hope this is at least a good start.

Should work on Apache (due to the changes in .htaccess in this branch) and also on GitLab pages (which looks for a 404.html). Not sure about surge. I guess we'll see when this builds.

Fixes #91 (closed).