task: Add Google Analytics (GA4) to docs.json

[GigaHierz]

Part of #2266.

What needs to be done?

docs.json has no analytics configuration. Verified at bdf40b37: "integrations" in docs.json → false; grep for ga4|gtm|googleAnalytics|posthog|amplitude|mixpanel|segment|plausible|fathom → 0 hits. Page analytics were lost with the Mintlify plan change and are one of the two highest-priority items right now (with #2250).

Mintlify's documented key path for Google Analytics 4 is:

"integrations": {
  "ga4": {
    "measurementId": "G-XXXXXXX"
  }
}

Definition of done

Ops (outside the repo):

• GA4 property created under a team-owned Google account (not a personal one); access shared with the DevRel team
• Owner of the GA account recorded in the PR description

Code:

• integrations.ga4.measurementId added to docs.json
• Verified after deploy: page views from docs.celo.org arrive in GA4 Realtime

How we'd know it's fixed

GA4 Realtime shows hits from docs.celo.org within 24h of merge.

Non-goals

GTM, cookie-banner changes, dashboards. The AI assistant is #2250.

Measured at: bdf40b37

[GigaHierz]

Done — PR #2267 merged as 453efdc2. GA4 is enabled via docs.json integrations.ga4.