Add troubleshooting section for gdk update

What does this merge request do and why?

Adds a troubleshooting section for gdk update, with details on how to troubleshoot network errors.

Impacted categories

The following categories relate to this merge request:

• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.

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.