[Feature Request] Add a custom 404 error page

It would be nice to have a custom 404 page, maybe with some example code and image from one of the pages.

There are a few issues relating to setting up 404 pages with Material for Mkdocs (and currently deploying our site through GitHub, though we may move off of GitHub!)

https://github.com/squidfunk/mkdocs-material/issues/6725
https://github.com/squidfunk/mkdocs-material/discussions/7904