Fix failing test:docs_lint job

What does this Merge Request do?

This MR aims to fix the failing test:docs_lint CI job by:

• Move vale/markdownlint setup into Makefile so running lint tests is now simply make lint for both CI and for local dev 🙌
• Enables GDK_DEBUG env var support so we can enable it for all CI jobs for greater visibility
• Fixing any markdown offences detected

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.

Closes #899 (closed)