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/libstd/str.rs b/src/libstd/str.rs
index b105dd0ca5a..adb0c299876 100644
--- a/src/libstd/str.rs
+++ b/src/libstd/str.rs
@@ -37,7 +37,7 @@ fn main() {
     let borrowed_string1 = "This string is borrowed with the 'static lifetime";
     let borrowed_string2: &str = owned_string;   // owned strings can be borrowed
 }
- ```
+```
 
 From the example above, you can see that Rust has 2 different kinds of string
 literals. The owned literals correspond to the owned string types, but the