Update Markdown lint image and add internal link checking

What does this Merge Request do and why?

Following gitlab-docs!2367 (merged), I've updated the Markdown linting image for this project.

While here, I added https://www.npmjs.com/package/markdown-link-check to provide internal link checking also. We have an image for that, but it requires docs to be build for Nanoc, so was looking for a light-weight alternative for this project. The library seems well maintained.

This could be extended to external link checking, but such a check would need to be allowed to fail because success and failure can't be controlled within this project.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.