docs(dns): enhance full setup documentation for Cloudflare DNS

[saihgenessofficial-coder]

Summary:

• Clarify the "Full setup" guide for using Cloudflare as the authoritative DNS provider.
• Add explicit steps for switching nameservers, disabling DNSSEC at registrars, and verifying propagation using dig/whois.
• Include a short Namecheap example for common registrars.

What changed:

• Expanded the steps in "Full setup" to call out disabling DNSSEC before changing nameservers.
• Added verification commands (dig/whois) and a brief Namecheap walkthrough.
• Improved wording to reduce ambiguity for end users and support teams.

Why:

• Users have reported confusion and errors when updating nameservers while DNSSEC is still active at the registrar. These clarifications should reduce support requests and failed verifications.

How to test:

• Review the docs preview and ensure code blocks render correctly.
• Confirm links and example registrar steps are clear and vendor-agnostic.
• Optionally validate that commands are accurate by running the provided dig/whois checks against a test domain.

Notes:

• This PR contains documentation changes only and no sensitive information.
• I have enabled "Allow edits by maintainers" to make minor follow-up edits easier.