Check markdown rules in github actions

This way we'll notice syntax inconsistencies in pull requests. We
weren't doing it until now because there's no Pronto runner for MDL and
we weren't running linters individually.

Note we aren't checking the templates in the `.github` folders, since
these templates don't have a top level header in the beginning (they'll
be displayed on a page which already has a top level header) and MDL
doesn't provide a way to ignore certain rules on certain files. So we're
specifying which files to include instead of running `mdl .` (which
would also include the `node_modules` folder) or running `mdl -g .`
(which would exclude the `node_modules` folder but would include the
`.github` folder).

.github/workflows/linters.yml

@@ -19,6 +19,8 @@ jobs:
             run: npx eslint . --quiet
           - name: Stylelint
             run: npx stylelint app/assets/stylesheets
+          - name: Markdownlint
+            run: bundle exec mdl *.md docs/
     name: ${{ matrix.name }}
     steps:
       - name: Checkout code