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/on-the-web.md

@@ -4,6 +4,7 @@ Here is a list of places mgmt has appeared on the web. Feel free to send a patch
 if we missed something that you think is relevant!
 
 ## Links
+
 | Author | Format | Subject |
 |---|---|---|
 | James Shubin | blog | [Next generation configuration mgmt](https://purpleidea.com/blog/2016/01/18/next-generation-configuration-mgmt/) |