…artifact

The "default" action now supports downloading from different workflow runs, too, so
we might as well use it.

…ng main

The previous setup would cause multiple commits on main to be stuck in a pending
state, waiting for the previous run to be finished.

The new group specification is taken from:
https://docs.github.com/en/actions/using-jobs/using-concurrency#example-using-a-fallback-value

This also adds the same settings for the docs workflow.

While this duplicates code a little bit, it makes it much simpler to refactor later on.

Both jobs run really quick, running them separate is just a waste of resources. Plus,
they are semantically closely related anyway.