docs(skaha): document metricsBackend and Skaha integration (CADC-15588)

[shinybrar]

Summary

• Expand metricsBackend helm-docs comments in skaha/values.yaml for deployers: opt-in defaults (enabled and rbac.enabled both false), in-cluster SKAHA_METRICS_BACKEND_URL behavior, required METRICS_* env from GitOps, Redis wiring, and optional ingress.
• Add README sections on Skaha ↔ Metrics integration (platform stats, shared ServiceAccount, metrics-server for session listings).
• Regenerate chart READMEs via helm-docs (includes minor table refresh on cavern and science-portal from the hook run).

Context

Supports CADC-15588: Skaha platform stats consume the co-deployed Metrics API when metricsBackend.enabled is true. Documents that metricsBackend.rbac.enabled stays false by default (CANFAR pre-provisions Kueue read on the Skaha ServiceAccount).

Test plan

• Review helm/applications/skaha/values.yaml comments and README prose for accuracy
• helm template with metricsBackend.enabled: true and sample env from keel-deploy overlay
• Confirm no template changes in this PR (docs only)

[github-actions]

✅ All pre-commit checks passed

Thanks for keeping the repo tidy! ✨