Signed-off-by: David Bauer <mail@david-bauer.net>

This allows us to organize the TOC a bit better by adding sections per
major version. We can even increase the maxdepth to 2 now, which looks
great in my opinion.

In addition, the full list of releases is not shown in the sidebar
anymore when viewing a completely different part of the documentation,
which took up more than half of the total sidebar entries.