Document and operationalize the existing fallout.build domain

## Why
`fallout.build` is already registered (Cloudflare Registrar) and serves a holding page from `gh-pages`. The project doesn't need to *acquire* a domain — but it still needs to:
- move off a personal Cloudflare account ([[Migrate Cloudflare account from personal to org]] — #234)
- get proper org-level config once the GitHub org exists (DNS verification, email, etc.)
- document state, owner, and renewal so it doesn't quietly lapse

Supersedes #184, which assumed the domain still needed registering.

## Current state (snapshot — verify before relying on it)
- **Domain:** `fallout.build`
- **Registrar:** Cloudflare Registrar
- **DNS:** same personal Cloudflare account; `A` records → GitHub Pages (`185.199.108-111.153`)
- **TLS:** Let's Encrypt, auto-provisioned by GitHub Pages
- **Content:** holding page (`index.html` on `gh-pages`)
- **Cost:** ~USD $25.20/yr at renewal

## Action
- [ ] Move the domain + DNS zone to the org Cloudflare account (delegated to #234)
- [ ] Add a DNS TXT record to verify the domain against the Fallout GitHub org (once [[Create Fallout GitHub organization and transfer repositories]] is done) — unlocks the verified-org badge
- [ ] Once an org inbox exists ([[Establish org-owned accounts for project communications]]): configure MX/SPF/DKIM/DMARC for outbound mail
- [ ] Replace the holding page with the real docs site (separate effort — link the docs-site issue here when filed)
- [ ] Document renewal owner, billing contact, expiry date, and recovery process in the maintainer-only credential store
- [ ] Set a reminder N days before expiry — silent lapse is a textbook footgun

## Coordinate with
- [[Migrate Cloudflare account from personal to org]] — the actual transfer happens there
- [[Create Fallout GitHub organization and transfer repositories]] — domain verification is per-org
- [[Establish org-owned accounts for project communications]] — supplies the inbox for MX/SPF
- [[Establish nuget.org organization account for Fallout.* packages]] — package metadata should point at `https://fallout.build`

## Done when
- Domain lives on the org Cloudflare account (via #234)
- Domain is verified against the Fallout GitHub org
- DNS configured for project email (if/when an inbox exists)
- Renewal owner, expiry, and recovery documented; expiry monitoring in place
- Holding page replaced with the real docs site (or a tracking issue exists)