Only markdownlint doc/**/*.md files for now

What does this Merge Request do?

Only markdownlint doc/**/*.md files for now as markdownlint doesn't currently support multiple paths to be specified.

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.