Currently broken links lead to a ReadtheDocs "Maze found" page.

This happens especially when a page does not exist in one of the doc version (e.g. when it refers to a removed feature).

We need a custom 404 page that would be:

• branded to our product
• contain a search box and a link to the front page

ReadtheDocs has a custom 404 functionality, but it does not work out of the box: https://docs.readthedocs.io/en/latest/hosting.html#custom-not-found-404-pages

If this turns out to be troublesome, we can consider at least customApache redirect to the front page.