This primary fix brought on by this upgrade is the proper matching of the ```
and ~~~ doc blocks. This also moves hoedown to a git submodule rather than a
bundled repository.
Additionally, hoedown is stricter about code blocks, so this ended up fixing a
lot of invalid code blocks (ending with " ```" instead of "```", or ending with
"~~~~" instead of "~~~").
Closes#12776
The changes are basically just because rustdoc runs tests/rendering on
more snippets by default (i.e. everything without a `notrust` tag), and
not anything significant.
