docs: add docs folder (architecture, developer, usage); update README; wire probe TTL cache + concurrency notes into docs

docs/usage/quickstart.md

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+# Quick Start
+
+## Compose
+```bash
+docker compose up -d --build
+```
+
+Full stack with observability:
+```bash
+docker compose -f docker-compose.yml -f observability/docker-compose.yml up -d --build
+```
+
+## Local Dev (minimal)
+- Start NATS locally
+- Run services:
+```bash
+cargo run -p aggregate
+cargo run -p projection
+cargo run -p runner
+cargo run -p gateway
+```
+
+## Verify
+- Gateway: GET /health, /ready, /metrics
+- Projection: GET /health, /ready, /metrics
+- Runner: GET /health, /ready, /metrics