Fixes #37434 - Use nightly for develop manual links

[ekohl]

Foreman has for the longest time had https://theforeman.org/manuals/$next redirect to https://theforeman.org/manuals/nightly in our Apache configuration. This means that on every branching we need to modify this. It also doesn't play well with caching and using pure static webservers.

If Foreman uses nightly as the version, this whole redirect is no longer needed.