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What does this merge request do and why?

A docs update for users new to rails and GitLab and unable to log in as root. Initially I did not have this problem, and logged in as root just fine. But on a subsequent rebuild I couldnt log in... potentially a problem of my own doing, but a quick fix was hard to find without a bit of ruby and ruby console knowledge.

How to set up and validate locally

These are just docs changes.

I was unable to log in as root, despite rebuilding with gdk install and gdk pristine, so I have to educate myself a bit on rails as a complete newbie.

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.

This should make it easier for community contributers to get started.

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.