RHCLOUD-47190: add tutorial framing to Quick Start and new Add Kessel to Existing App tutorial

[tonytheleg]

• Update Quick Start to add time estimate and "What you learned" summary (Diataxis alignment)
• Adds new "Add Kessel to an existing application" tutorial under Start Here with a 5-step walkthrough (map resources, design schema, set up roles, report resources, protect endpoints)
  • New tutorial cross-references how-to guides without duplicating content and expands on whats learned in quick start

Full tutorial validated locally

# validation by showing last step returns ALLOWED_TRUE
$ grpcurl -plaintext -d '{
  "object": {
    "resource_type": "task",
    "resource_id": "task-001",
    "reporter": {
      "type": "TASKMANAGER"
    }
  },
  "relation": "view",
  "subject": {
    "resource": {
      "resource_type": "principal",
      "resource_id": "user-123",
      "reporter": {
        "type": "rbac"
      }
    }
  }
}' localhost:9081 kessel.inventory.v1beta2.KesselInventoryService.Check
{
  "allowed": "ALLOWED_TRUE",
  "consistencyToken": {
    "token": "GhAKBENJc0gSCGUyNjJjZGUw"
  }
}