docs: add product overview and engineering documentation

Document architecture, security, deployment, and development for contributors.

docs/README.md

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+# Pargar documentation
+
+This folder is the engineering source of truth. [`README.md`](../README.md) is only the entry point; details live here.
+
+## Reading map
+
+1. **Newcomer / product** → [product.md](product.md)
+2. **Architecture** → [architecture.md](architecture.md)
+3. **Local development** → [development.md](development.md)
+4. **Deployment** → [deployment.md](deployment.md) (plus the ops shortcut in [`deploy/README.md`](../deploy/README.md))
+5. **Security & launch** → [security.md](security.md)
+6. **Day-to-day ops** → [operations.md](operations.md)
+7. **API contract** → [api.md](api.md)
+8. **Env & settings** → [configuration.md](configuration.md)
+
+## Writing conventions
+
+- File paths, env names, shell commands, and route names stay in **exact English**.
+- Prefer linking over long duplication across files.
+- When behavior changes, update the matching doc in the same PR.
+
+## Related files outside `docs/`
+
+| File | Role |
+|------|------|
+| [`CONTEXT.md`](../CONTEXT.md) | Short domain summary for agents / fast onboarding |
+| [`deploy/README.md`](../deploy/README.md) | Deploy command shortcut |
+| [`.env.example`](../.env.example) / [`.env.prod.example`](../.env.prod.example) | Secret templates |