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).

[coolharsh55]

Please review an implementation at https://harshp.com/dpv-x/test which:

1. provides url and w3id purl in case there are typos
2. checks if the url is unversioned and provides a versioned form (for latest release 2.0) to check if that is the correct url
3. provides a list of resources (directories) in the repo which correspond to specifications

Code is at https://github.com/coolharsh55/dpv-x/blob/master/404.html

[coolharsh55]

Completed - see https://w3id.org/dpv/404/404 for demo.