Problem

docs/internals/release-process.rst explains the concept of the Django fork and its branch structure, but does not show:

1. The current active mapping between django-mongodb-backend branches and their corresponding Django fork branches (mongodb-forks/django)
2. The pre-release branch naming convention (django61, django62) used for "Update to Django X.Y" pull requests

A contributor has no way to know which fork branch corresponds to their working branch, or why django61/django62 branches exist.

Work

Add to docs/internals/release-process.rst:

• A table mapping each active django-mongodb-backend branch to its Django fork branch (e.g. main → mongodb-6.0.x, 5.2.x → mongodb-5.2.x, django61 → mongodb-6.1.x)
• A description of the pre-release branch convention (djangoXY branches used for "Update to Django X.Y" PRs) in the branch policy section