It's not actually a reasonable experience to use Zulip without working
full-text search, so we should more strongly direct folks to not do
it.
Fixes#12724.
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.
Tim Abbott
Samuel Searles-Bryant
Igor Posledov
Bruce