docs: add the App Analytics guide

[ditadi]

Why the change

App Analytics now spans an SDK, a server relay, appkit-ui exports, and an auto-start script, so this adds one docs section that explains it as a whole, both for AppKit apps and for apps built without AppKit.

Special things to note

• Docs only, written against the final behavior of feat: add the @databricks/app-analytics browser SDK #611–feat(appkit): start App Analytics automatically when App telemetry is on #614 so reviewers read each page once, not in versions that later PRs rewrite.
• architecture.md includes a reference relay for apps built without AppKit. It mirrors the behavior of the built-in relay from feat(appkit): relay App Analytics records to the OTel Collector #612: size and content-type limits, the 204 when telemetry is off, and the Collector's status passed back.

Change outline

 docs/docs/
+├── app-analytics/
+│   ├── _category_.json
+│   ├── getting-started.md     # set up collection, first Action, verify, what must never be collected
+│   ├── using-with-appkit.md   # auto-start, <AppAnalytics />, the relay's answers, turning it off,
+│   │                          #   enabling App telemetry, the dev playground page
+│   ├── events.md              # Action, Page View, Web Vital, properties, what to track
+│   ├── api-reference.md       # data specification: event schema, otel_logs encoding, privacy, compatibility
+│   └── architecture.md        # pipeline, delivery, sessions and sampling, reference relay without AppKit
 └── plugins/
     ├── server.md              # + POST /_analytics/v1/logs, appAnalytics and bodyLimit options
     └── analytics.md           # + note: SQL analytics plugin ≠ App Analytics

This pull request and its description were written by Isaac.