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/faq.md

@@ -1,4 +1,4 @@
-# Frequently asked questions
+## Frequently asked questions
 
 (Send your questions as a patch to this FAQ! I'll review it, merge it, and
 respond by commit with the answer.)
@@ -98,6 +98,7 @@ shutdown after that time interval using the `--max-runtime` flag. This also
 requires a number of seconds as an argument.
 
 #### Example:
+
 ```
 ./mgmt run --lang examples/lang/hello0.mcl --converged-timeout=5
 ```