Draft: Switch docs to use main as default branch

What does this Merge Request do?

Draft until all the projects involved have renamed their default branch to main

All mentions to master have been replaced with main, because that's the default branch now.

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.