Provide a 404 page for DPV

[coolharsh55]

The 404 page currently is the generic Github 404 page e.g. https://w3c.github.io/dpv/random

Create a new 404 page for DPV that displays the 404 error, and (optionally) gives a list of all DPV links (e.g. all paths leading to HTML docs) or a broad overview of DPV with links (e.g. specs and extensions) or link to a sitemap (if sitemap is required, then create a new issue to generate the sitemap).