Make 404 page consistent with main beeware.org website

[freakboy3742]

Describe the bug

The 404 page for docs-tools isn't a complete match for the 404 page on the main BeeWare website - the body content is very similar, but its missing the header and some other styling.

Steps to reproduce

1. Go to https://beeware-docs-tools.readthedocs.io/en/latest/does-not-exist
2. Compare with https://beeware.org/does-not-exist

Expected behavior

Pages should be substantially the same style

Screenshots

No response

Environment

• Operating System: all
• Python version:
• Software versions:
  • Docs tools @ 20250822

Logs

Additional context

The complication is linking resources - the CSS and JS needed to display the default toolbar.

As the docs-tools site is multi-lingual, the root of the site is /en/latest; and serves assets as /en/latest/assets/... however the 404 page is trying to load assets from /assets/....

Two possible approaches:

1. Find out how to reference the "site root" as part of mkdocs, and use that in asset references in the head block
2. Reference the "live" versions of those resource in the head block. This would mean 404 resources are pointing at potentially stale resources; but it's a 404 page, so it's not critical that they're always live.

[johnzhou721]

FWIW: 404 images are actually missing images as well.

EDIT -- just realized you can't reproduce this with the link in this post. https://toga.readthedocs.io/en/latest/tutorial/tutorial-10 is a reproducer

[kattni]

There is an issue with how MkDocs and Read the Docs handle 404 pages that make this something we can't deal with right now. That's what is causing the images to appear to be missing. Unfortunately, we're going to have to deal with the current state of it until something changes in the tooling.

[johnzhou721]

The 404 page looks like this now.

See https://toga.beeware.org/en/latest/tutorial/dne