Document the Setup Ceremony Math

[cronokirby]

A good first step towards completing #712 is to document the proposed math we're planning on doing for the setup ceremony. This will give us a clear reference we (and others?) can use to implement the "isolated" part of the logic, pending further design discussion on the orchestration aspect of the ceremony.

This would also be a place to reference the papers we're implementing, and have some commentary on some of the security considerations guiding our choices there.

I think having this in our protocol docs make sense, as a new section.