Link to non-GDK instructions for setting up local preview of GitLab Docs

What does this merge request do and why?

Because some folks might come to the GDK documentation first, they might not know that you don't need to use GDK to contribute to GitLab Docs.

This MR provides a link to the "non-GDK instructions", so that folks don't set up GDK just for documentation if they don't need to.

CC @kpaizee @dianalogan

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

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 an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.