Add details of how to link local gitlab-ui package

What does this merge request do and why?

Adds a link to the docs for testing a local version of gitlab-ui with the GitLab project.

Adding link to this page as this is the first place I looked to try and do this.

Had to ask in Slack for an answer as I wasn't able to figure it out myself, hopefully adding it here makes others more efficient.

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.