Add docs-specific 404 page with search form

[elemasamuel]

Closes #1347

The 404 page currently shows a generic error message with no way to find
what the user was looking for. This is especially frustrating on the docs
subdomain where users often land via broken or outdated links.

This PR adds a docs-specific 404 template at docs/templates/docs/404.html
that includes the existing search form, so users can immediately search for
the content they need without leaving the page.

[elemasamuel]

Hi @adamchainz @pauloxnet , could you please take a look at this PR when you get a chance? It adds a docs-specific 404 template with the search form as discussed in #1347. Happy to make any changes needed.
Thanks! 🙏

[adamchainz]

Thanks for giving this a try. Can you share a screenshot?

[elemasamuel]

Hi! Here's a screenshot of the docs-specific 404 page in action:

Let me know if you'd like any changes!