Draft: Add developer documentation for Pull mirroring process

What does this MR do and why?

Contributes to #474118 (closed)

See changes in Review app

https://main-ee-172707.docs.gitlab-review.app/ee/development/backend/create_source_code_be/pull_mirroring.html

References

Please include cross links to any resources that are relevant to this MR This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• High level Architecture and Flow of Source Code (#474118 - closed)