The numbers are in parens to avoid the markdown processor interpreting them
as numbers -- I couldn't get the automatic numbering to work out with the
paragraph breaks and so on, and this is probably good enough for now.
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.
Now that we have nice documentation for our export/import tools, we've
been seeing a lot of users trying to use that as their primary backup
process. Let's correct this.