insights: Add docs around permissions

Created by: CristinaBirkel

Closes: https://github.com/sourcegraph/sourcegraph/issues/29530

Description

Docs only change. This adds some information to our docs that is intended to be useful to the security team to answer questions about how code insights permissions work.

I'm fairly unsure that I addressed the right issues here, so please let me know if I missed important details that we need to share!