Update docs lint image and add Markdownlint configuration

What does this Merge Request do?

Doesn't require that GDK documentation follow rules we have in place for "published docs", but points a WIP docs job to the image that would be required, and adds Markdownlint configuration so that documentation can begin to conform (opt-in).

CC @marcel.amirault

Can decide down the track if GDK documentation should be required to conform.

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.