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.

Those are more future proof and will work in gitlab.

Those instructions are better in "Reference" because it's about the
current design, "how things work", not "how to use them".

We would still need a backup/restore howto section, that said.