[DNS] Document modern nameserver sets

[svenr-cf]

Summary

This documents "nameserver sets", a new way to manage Custom Nameservers in the DNS.

Screenshots (optional)

None.

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 change adheres to the documentation style guide.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.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.

[cloudflare-docs-bot]

AI Review

✅ Reviewed f014e97 · 0 findings · incremental

Resolved (2) · Dismissed (0)

• Resolved: Replace e.g. with for example · Verified at head: advanced-nameservers.mdx line 91 now reads '(for example, ns1.example.com)'; the e.g. abbreviation has been replaced, satisfying the core-content style rule.
• Resolved: Missing serial comma in three-item list · Verified at head: line 93 now reads 'tenant, account, or zone' with the serial comma present before the final or, satisfying the core-content serial-comma rule.

Commands

• /review Run a review now.
• /full-review Review the entire PR diff.
• /disable-auto-review Stop automatic reviews.
• /rebase Rebase against production.

[mvvmm]

Any reason this change is included?

[svenr-cf]

Yeah, the OpenAPI was only published recently on our side, and <APIRequest requires it