Add First-Party Domains documentation

[micaww]

Adds a public docs page for First-Party Domains (Enterprise), the managed alternative to running your own Mixpanel proxy.

• New page: docs/tracking-methods/first-party-domains.mdx — overview and comparison vs. self-hosted proxy, requirements, DNS setup, status reference, per-SDK host config, supported endpoints, deletion grace period, troubleshooting, FAQ
• Registered in docs.json nav under Data In → Tracking Methods
• Cross-linked from the JavaScript SDK "Tracking Via Proxy" section

Content verified against the implementation: the api-proxy GCP load balancer URL map (which paths are served, and that cdn.mxpnl.com assets are not), the Enterprise plan gate, the 3-domain limit, the four certificate statuses, per-region CNAME targets, and the 5-day deletion grace period with cancel.

Note for a follow-up in the product repo: the in-app "View docs" links currently point at tracking-methods/sdks/javascript#step-1-set-up-a-proxy-server and should be repointed at this page once merged.

[mintlify]

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Project	Status	Preview	Updated (UTC)
mixpanel-docs	🟢 Ready	View Preview	Aug 21, 2026, 5:42 PM

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[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge with no actionable issues identified.

The new page is correctly registered, its internal routes and anchors resolve, and its SDK configuration examples match the existing platform documentation.

Important Files Changed

Filename	Overview
docs.json	Registers the new page using the established navigation path convention.
docs/tracking-methods/first-party-domains.mdx	Adds comprehensive First-Party Domains setup and reference documentation with internally consistent SDK examples and valid links.
docs/tracking-methods/sdks/javascript.mdx	Cross-links proxy users to the managed First-Party Domains alternative.

_{Reviews (1): Last reviewed commit: "Describe First-Party Domains as generall..." | Re-trigger Greptile}