Add doc for consent proof signature prompt

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Name	Status	Preview	Comments	Updated (UTC)
junk-range-possible	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jun 6, 2024 2:03am

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Deploying xmtp-org with    Cloudflare Pages

Latest commit:	00fa5e4
Status:	✅  Deploy successful!
Preview URL:	https://7d084f43.website-9re.pages.dev
Branch Preview URL:	https://consent-sig.website-9re.pages.dev

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[jhaaaa]

Delivering in https://github.com/xmtp/xmtp-dot-org/pull/757 instead. Closing this PR.