Tools for testing documentation

Like code, documentation needs some ❤ and proper testing.

As a first step, we need to see that a couple of things are properly handled:

• Markdown formatting (we can use markdownlint) - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4900
• Links should always point to a valid location https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9509
• Spell checking. This should be better tackled locally by one's editor, but we could at least check for words like GitLab to be spelled correctly.

Custom linting scripts:

• Add linting tool to use the long options in cURL examples https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5703