Provide ability to debug permissions using specs

What does this MR do and why?

• To my knowledge, there is no other way of programmatically knowing which permissions checks are called for a request.
• This approach inspired by !94135 (comment 1295929169)
• Also updates docs to explain usage
• And adds section on ability_map, which I used here: !113168 (merged)
• Also adds an RSpec shared example for comparing policies, which I used here: !112762 (closed)
• Prep work for "consolidate permissions" epic: #389631 (closed)

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