Add FAQ Item about admin roles

[mds1]

Question What admin roles does Umbra have?

Answer

• Umbra: Explain the powers of owner, tollCollector, tollReceiver, and the addresses they are set to on mainnet
• ENS: Explain any applicable ENS admin roles/contracts (can reference the ENS subdomain revoking question here if it helps)

@apbendi If you write the answer directly in this issue, I can migrate it to the frontend code