New developers joining the django-mongodb-backend project lack documentation explaining key concepts and context.

Create a technical onboarding guide covering:

• Django upstream — how the backend relates to upstream Django, what we track, and how changes flow in
• Architecture — overview of the backend structure and key components
• Extensions — custom Django extensions provided by the backend
• Workflows — development, testing, and release workflows
• Testing — how to run the test suite and write new tests
• Fork — how the Django fork is managed and kept in sync

The output will be a Google Doc (and potentially a docs page) that can be shared with new contributors and team members.