docs: add kagent-to-MLflow tracing quickstart

[hsteude]

Summary

• add a minimal OpenTelemetry Collector manifest that sends workspace kagent traces to MLflow
• document MLflow experiment, PAT, Secret, deployment, verification, GitOps, and cleanup steps in one quickstart
• mention that other OpenTelemetry-compatible backends such as Langfuse can use the same pattern
• link the tracing quickstart from the kagent examples index

Testing

• yamllint kagent/observability/mlflow-tracing/otel-collector.yaml
• kubectl apply --dry-run=client -f kagent/observability/mlflow-tracing/otel-collector.yaml
• Collector 0.159.0 validate command against the embedded configuration
• pre-commit run --files kagent/README.md kagent/observability/mlflow-tracing/README.md kagent/observability/mlflow-tracing/otel-collector.yaml
• live kagent request produced MLflow trace tr-0101febea78407a86e44d4e700405b46 with six spans and state OK

Dependency

• stacked on feat: add progressive and LangGraph BYO kagent examples #62 because that PR introduces the kagent/ examples tree
• related platform prerequisites: prokube/mlflow-oidc-auth#9 and prokube/prokube#401

UI Evidence

UI evidence: not required
Reason: documentation and example Kubernetes manifests only; no intentionally visible product UI output changes.

Closes #64