Add instructions to customize terminal with branch name

What does this merge request do and why?

Update the gdkiab docs to include instructions on how to customize the terminal with colors and branch name. This was another friction point for me - I found myself constantly running git branch to make sure I wasn't committing to the wrong place. With the branch name clearly in the prompt, that's easily avoided!

Before	After

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.