test, docs: Add a linter for testing markdown, and fix up our docs

While writing docs, I couldn't remember what the correct style was
supposed to be, and I remember someone complaining about this
previously, so I decided to add a linter! I excluded a bunch of annoying
style rules, but if we find more we can add those to the list too.

Hopefully this gives us a more consistent feel throughout.

docs/style-guide.md

@@ -91,5 +91,6 @@ When implementing code for the various types in the language, please follow this
 order: `bool`, `str`, `int`, `float`, `list`, `map`, `struct`, `func`.
 
 ## Suggestions
+
 If you have any ideas for suggestions or other improvements to this guide,
 please let us know!