Clarify Source Code BE Processes

Why is this change being made?

During an engineering manager office hours call, I recommended that it's super useful to look at the handbook pages of other groups for process-related information, since GitLab doesn't have strict processes around interacting with counterparts or how to do certain things as a manager. It's really confusing as a new engineering manager, because GitLab has so much documented, so you feel like you're missing something when team processes are up to you - especially since there are so many teams and you don't want to duplicate effort. In doing so, I realized that I haven't updated our handbook on any of these areas (I meant to do it!)

These are the same processes we've been following, I just documented them for others.

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what