Adding instructions to configure the second cell to use gdk.test

What does this merge request do and why?

If you configure GDK to use the gdk.test url, you need to configure the second Cell to also use gdk.test so the two instances share logins. So this MR adds a link to the instructions to setup gdk.test for the second Cell.

Related issue

• 2 Cells running in GDK with different domains d... (gitlab#426003 - closed)

How to set up and validate locally

1. Setup initial GDK with the http://gdk.test:3000 URL
2. Follow the instructions to setup a second Cell
3. Launch both Cells
   1. Log into the first Cell http://gdk.test:3000
   2. Visit second Cell http://localhost:3001, you are not logged in
4. Configure second cell for gdk.test

   cd <gdk2 directory>
   gdk config set hostname gdk.test
   gdk reconfigure
   gdk restart

5. verify that when you visit http://gdk.test:3001, you are logged in.

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.