Improve runner setup docs

What does this Merge Request do and why?

Two people in the last week have struggled to set up a runner with the GDK. Both followed the GitLab Runner documentation for setting up a runner inside a Docker container. This made things more complicated because they then had to juggle hostname, networking, and SSL certificate management. Plus, it's not always clear where the config file is stored.

The GDK has built-in support for managing the runner config and process, and we should encourage users to stick to this.

This documentation removes links to the GitLab Runner documentation and tries to make it clearer that the GDK can do the heavy lifting.

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