Lint markdown

We should use [markdownlint-cli](https://github.com/igorshubovych/markdownlint-cli), but they don't seem to support glob patterns so we can't ignore `node_modules` ([opened a ticket](https://github.com/igorshubovych/markdownlint-cli/issues/16)).

Another option is Gil's [check-all-markdown](https://github.com/create-oss/check-all-markdown), but it has no flag to run just the Markdown lint without the URL checks ([opened a ticket](https://github.com/create-oss/check-all-markdown/issues/2))

Or we can write our own package to just lint Markdown with markdownlint-cli using [globby](https://github.com/sindresorhus/globby) to support normal glob matching and ignore `node_modules` and `.git` folder by default.