Document which Datadog products are configured and what to expect

[scottgerring]

Problem

The README says Stickerlandia is "fully instrumented with Datadog's observability and analysis tooling" but never specifies which products are configured or what data users should expect to see. This led to confusion about whether Data Streams Monitoring (DSM) was working — DD_DATA_STREAMS_ENABLED=true is set on all services, but it's unclear which deployment modes support it and whether the user's Datadog org needs specific features enabled.

Suggested Fix

Add an "Observability" section to the docs that explicitly covers:

• Which Datadog products are configured (APM, Logs, RUM, DSM, Profiling, IAST, Runtime Metrics)
• Which deployment modes support which features (e.g. DSM requires Kafka-based messaging — works in docker-compose and EKS, but not in the serverless/EventBridge AWS deployment)
• Any prerequisites on the Datadog org side (e.g. DSM must be enabled in your account)
• What dashboards/views to check for each product

Part of #235