docs: add integrations landing page for operator guides (#1043)

## Summary

- Adds `docs/operator-guides/integrations/index.md` as a navigation hub
for all integration content
- Organizes guides into three sections: Built-in Components (Model
Registry), External System Integrations (Experiment Tracking), and
Extending Built-in Components (Custom Serving Backend, Custom Scheduler,
Register a Compute Cluster)
- Links back to related platform guides via a "Next Steps" section
(auth, network, monitoring, troubleshooting)

## Why

The individual integration guides exist, but without a landing page
operators have no single place to discover all integration surfaces.
This completes the integrations section started by #1036 (operator index
restructure).

## Test plan

- [x] All relative links resolve (verified after #1041 and #1042 merged
from main)
- [x] Hub matches voice and structure of sibling index pages
(`jobs/index.md`, `serving/index.md`)

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: Austin Greco <austingreco@gmail.com>
Co-authored-by: Austin Greco <agreco@uber.com>

Author: 3 people

docs/operator-guides/integrations/index.md

@@ -0,0 +1,37 @@
+# Integrations
+
+Michelangelo runs alongside the ML infrastructure your organization already has. This page is the operator reference for connecting external systems and extending Michelangelo's built-in components. It targets platform engineers responsible for running Michelangelo in production.
+
+## Built-in Components
+
+These guides cover components that ship with Michelangelo. Operators configure them; they are not external systems.
+
+| Guide | Description |
+|-------|-------------|
+| [Model Registry](model-registry.md) | Verify the registry is healthy, configure object store and RBAC, and integrate registered models with serving and CI/CD |
+
+## External System Integrations
+
+These guides cover connecting Michelangelo to systems your organization already runs.
+
+| Guide | Description |
+|-------|-------------|
+| [Experiment Tracking](experiment-tracking.md) | Expose an external experiment tracking server to task pods — network setup, URI injection, and operator/user boundary |
+
+## Extending Built-in Components
+
+Michelangelo exposes extension points for replacing or augmenting its core subsystems. Use these when the defaults don't fit your infrastructure.
+
+| Guide | Description |
+|-------|-------------|
+| [Custom Serving Backend](../serving/integrate-custom-backend.md) | Add support for any inference framework — Triton, vLLM, TensorRT-LLM, or your own |
+| [Extend the Job Scheduler](../jobs/extend-michelangelo-batch-job-scheduler-system.md) | Replace or extend the scheduler — integrate Kueue, Volcano, or implement a custom `JobQueue` and `AssignmentStrategy` |
+| [Register a Compute Cluster](../jobs/register-a-compute-cluster-to-michelangelo-control-plane.md) | Connect an existing Kubernetes cluster so Michelangelo can dispatch Ray jobs to it |
+
+## Next Steps
+
+- [Platform Setup](../platform-setup.md) — ConfigMap reference for all components
+- [Authentication](../authentication.md) — OIDC, RBAC, and service-to-service auth
+- [Network & Ingress](../network.md) — Ingress setup, Envoy proxy config, TLS, multi-cluster networking
+- [Monitoring](../monitoring.md) — Prometheus metrics, alerting, Grafana dashboards
+- [Troubleshooting](../troubleshooting.md) — Common failure modes and `kubectl` diagnostic commands