docs(example-doc-vault): write comprehensive step-by-step tutorial and README

[crapougnax]

Part of parent story #36.

🎯 Goal

Author the comprehensive intermediate-level tutorial for the Document Vault reference application.

📋 Specification

1. Create examples/document-vault/README.md detailing:
   • Architecture overview: Dual-database persistence, FSM engine, and RBAC governance.
   • Setup instructions (Docker Compose with PostgreSQL + MinIO).
   • Step 1: Defining the Document model & FSM workflow.
   • Step 2: Registering PostgreSQL (default) and SQLite (archive) backends.
   • Step 3: Demonstrating automatic ObjectUri query and write routing.
   • Step 4: Protecting sensitive properties with Field-Level Security (FLS).
   • Step 5: Uploading and downloading assets via S3 presigned URLs.
2. Cross-link tutorial in docs under docs/pages/tutorials/document-vault.md.