Update docs to ensure we refer to gdk.test

Overview

We have multiple references within the GDK docs for what name to give the default host, gitlab.local, gdk.local and gdk.test (as mentioned at !1191 (comment 330735545))

Proposal

As discussed at !1191 (comment 330735545), .test was determined to be the most appropriate domain going forward but we still have a mix of gdk. and gitlab. throughout the docs.

Let's standardise on the default being gitlab.test.