Codeberg is a Gitea-based git hosting service that uses the same URL
patterns for pull requests and commits. This adds native support so
users don't need to manually configure it.
Introduce the 'SelectedSubmodule' struct and allow using it as a
placeholder value.
Add a corresponding test.
Update the documentation to include it among the listed placeholder
values.
The plan is to keep the original docs and schema folders unchanged for the
duration of a release; we'll only continuously update the -master copies. Right
before a new release we will copy them over.
