Sphinx/ReadTheDocs supports automatically translating links written as
to `.md` files to point to the corresponding `.html` files, so this
migration does not change the resulting HTML output in ReadTheDocs.
But it does fix apparent broken links on GitHub.
This doesn't prevent people from reading the documentation on GitHub
(so doesn't mitigate the fact that some rtd-specific syntax does not
render properly on GH), but it will prevent us from getting erroneous
issues reported about the hyperlinks not working.
Fixes: #11087.
This fixes an actual user-facing issue in our mobile push
notifications documentation (where we were incorrectly failing to
quote the argument to `./manage.py register_server` making it not
work), as well as preventing future similar issues from occurring
again via a linter rule.