Use 'nanoc live' to speed up gitlab-docs start

What does this merge request do and why?

This MR suggests to use nanoc live over nanoc view to speed up the gitlab-docs start.

While this leaves most docs pages in a 404 in the beginning, it will compile incrementally.

How to set up and validate locally

1. Run gdk config set gitlab_docs.enabled true
2. Run gdk reconfigure
3. Run gdk start
4. Open the docs URL in your browser (see the ready message for the correct URL)
5. See that it is available after a few seconds instead of 2-3 minutes later

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.