Clarify Cloudflare DNS setup for custom domains

[qayshp]

Summary

• clarify that Cloudflare needs the root domain while BlueBubbles can use a subdomain
• replace the missing “records below” instruction with a DNS migration checklist
• explain that a public hostname normally creates its tunnel CNAME automatically
• document the complete <tunnel-UUID>.cfargotunnel.com target for the manual fallback

Why

The guide told readers to copy records that were never listed, which could leave existing websites or email records behind during the nameserver change. It also told readers to use a bare tunnel ID as a CNAME target even though Cloudflare Tunnel targets use the cfargotunnel.com hostname.

User impact

Readers can move an existing domain to Cloudflare without silently dropping unrelated DNS services, and they can confidently use a dedicated BlueBubbles subdomain.

Checks

• git diff --check
• confirmed the obsolete “records below” and bare tunnel-ID instructions are gone
• compared the public-hostname behavior and CNAME target with Cloudflare's current Tunnel routing documentation

Fixes #25

[qayshp]

Self-review: the broken DNS migration instruction is fixed, with one wording edge case to address before this is ready.

[qayshp]

Validation update

• git diff --check origin/master...HEAD passes.
• Current Cloudflare documentation confirms that a tunnel receives a <UUID>.cfargotunnel.com hostname and that the public hostname uses a CNAME pointed at that full target.
• The obsolete “records below” wording and bare tunnel-ID target are absent from the updated guide.

A live Cloudflare dashboard/tunnel test was not performed, so this pass does not independently verify the dashboard's current automatic DNS-record creation behavior.