Add markdown linter to CI

[ceddlyburge]

It would be good to lint the markdown in CI, its very hard to spot mistakes without releasing and checking on the live website. It would also help with the annoying thing where every sentence is supposed to be on its own line.

There are lots of markdown linters out there, and presumably they check that links are formatted correctly.

It would be good to be able to run the linter locally as well.