Added ActiveRecord::PendingMigrationError section

What does this Merge Request do?

I added a section describing how I resolved a blocking error whilst setting up the gdk. I'm not certain why I encountered this error in the first place but it took me a while to figure out the solution, so I added it to the troubleshooting doc.

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.