Create a custom 404 page for rafiki.dev

[JoblersTune]

Since the docs structure has gone through an overhaul, we'd like to gracefully fail and redirect people to the home page when they click broken links.

This involves creating a custom 404.astro file in packages/documentation/src/pages and including either a button or an automatic redirect that guides users back to the home page.

[melissahenderson]

Completed with #2953