Cloudflare Registrar mention in onboarding.mdx

[rawalc]

Cloudflare offers a Registrar. Why point to external services and cause confusion rather than point to the Cloudflare Registrar directly, already.

Summary

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Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

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