Create custom 404 page

[cw118]

Suggestion/new feature

Rather than using the default GitHub Pages 404 page, a custom 404 page should be created in 404.html (place in root level of the repo) to avoid loss of traffic and confusion for users. Consider including a sitemap (possibly formatted as a table) in the main content container.

Motivation

Allows users to navigate the site more easily/find what they're looking for if an erroneous link is requested (likelihood is higher after site/directory reorganization).