Improve 404 Page and Dead Link Handling

[montymxb]

After re-designing the layout of the documentation, we've ended up with a number of dead links. For example, the old getting-started link now redirects to a 404. This in itself is somewhat okay, but the 404 page is very broken looking, as can be seen below.

The 404 page template needs to be corrected, and in cases where we know a link has moved, and it is appropriate to redirect to the new link location, we should do so.

Originally mentioned in eclipse-langium/langium#1512

[Lotes]

Can be fixed as part of #235