Add custom roles contribution docs

What does this MR do and why?

It adds guidance on how to develop custom roles. It also refactors the Permissions doc in general into subpages (simple copy & paste).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #393931 (closed)