rustdoc: Migrate from sundown to hoedown

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

1 files changed, 1 insertions, 1 deletions

diff --git a/src/libhexfloat/lib.rs b/src/libhexfloat/lib.rs
index 72528e96917..100bec23775 100644
--- a/src/libhexfloat/lib.rs
+++ b/src/libhexfloat/lib.rs
@@ -27,7 +27,7 @@ extern crate hexfloat;
 fn main() {
     let val = hexfloat!("0x1.ffffb4", f32);
 }
- ```
+```
 
 # References