Add developer docs for RBAC

[courier-new]

We've added user-facing documentation for RBAC for site admins to understand how to effectively use it with their userbase. Now, we should add internal developer documentation for understanding how the RBAC system works and how to expand it to other areas of the product. We should consider as part of this work how we will encourage adoption by other teams and "graduate" RBAC from being owned by Batch Changes.

Docs should include:

• Technical overview of the implementation (packages, DB schema overview, permissions schema file, startup process, etc.)
• How to add new permission namespaces with code examples
• Future plans for scoping to entities

[github-actions]

Hey, @sourcegraph/batchers (@eseliger @courier-new @adeola-ak @BolajiOlajide @Piszmog @malomarrec @chrispine @danielmarquespt) - we have been mentioned. Let's take a look.