Since the project was archived, the group wasn't showing issues...

we skip the `Workflows` section since it doesn't seem well defined yet
and somewhat project specfic. The Structure/Groups section also seemed
redundant with the Content organisation section.

We also skip the exhaustive "Team organisation" list and rely on the
actual `.txt` file for the components mapping instead of inlining it.

We do not trust that this external link will survive forever, and
besides it is better if it lives here, alongside the migration
documentation.

It seems refered to by the .setup file, damnit.

This reverts commit c714a045.

This is a test to see if it's worth doing by hand.

We also rephrase some things and fix a broken link (git) while we're here.

The latter is actually one of two standard file extensions (as per
RFC7763, the other being .markdown). It's also the extension expected
by GitHub, GitLab, and other wiki software.

So for the sake of future-proofing and compatibility, rename all files
to the new name.

This will hopefully not break anything: the file extension is not
present in the rendered version anyways.