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.

test.sh

@@ -48,6 +48,7 @@ run-testsuite ./test/test-gofmt.sh
 run-testsuite ./test/test-yamlfmt.sh
 run-testsuite ./test/test-bashfmt.sh
 run-testsuite ./test/test-headerfmt.sh
+run-testsuite ./test/test-markdownlint.sh
 run-testsuite ./test/test-commit-message.sh
 run-testsuite ./test/test-govet.sh
 run-testsuite ./test/test-examples.sh