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/libfourcc/lib.rs b/src/libfourcc/lib.rs
index fe0b6bcfd0f..04fa139168f 100644
--- a/src/libfourcc/lib.rs
+++ b/src/libfourcc/lib.rs
@@ -31,7 +31,7 @@ fn main() {
     let little_val = fourcc!("foo ", little);
     assert_eq!(little_val, 0x21EEFFC0u32);
 }
- ```
+```
 
 # References