Run doc lint tests the simple way

What does this Merge Request do?

Runs the docs lint tests in the simplest possible way, hopefully removing any possible problems with dependencies or make config. This should keep the linting in a stable state that won't be affected by any changes except when the lint image is updated (by updating the image: parameter in the pipeline).

The docs-lint image is already fully prepared for linting and needs no further config, so we can just run the two CLI commands for vale and markdownlint, and that's all that needs to be done.

Closes #1057 (closed)

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.