Document CODEOWNER and Approval Rule supported configurations

Problem to Solve

Currently CODEOWNERS and Approval Rules have many different items that need to be true in order for the approval rules to apply. This leads to some confusing situations where private groups can sometimes be seen in certain situations but maybe not to everyone. Or members of inherited groups might not be eligible to approve, but instead need to be invited directly.

All of these situations are confusing and we don't have a centralized place to reference and define what the expectations actually are.

Proposal

We should create technical documentation that outlines all the situations and configurations to better understand the current situation. Then we can use it as a framework for improvements and changes to these rules.

Additional Details

Below is just an example, the table IS NOT accurate:

	CODEOWNERS	Approval Rules
Inherited membership	yes	no
Private group visibility in merge request widget	yes	yes
Private group visibility in API response	no	yes
some feature/visibility	yes	no