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.
help.github.com seems to have a bug where HEAD on a redirected page
returns 404. This causes tools/test-documentation to fail. Fix it by
skipping the redirects.
Signed-off-by: Anders Kaseorg <andersk@mit.edu>
