Add example values files and an ArgoCD Application example

Checklist items:

[A] At least one example values file that deploys without modification (other than hostname)
[B] Example values file for full Nebari deployment (nebari-values.yaml)
[B] If standalone-supported: Example values file for standalone deployment (standalone-values.yaml)
[B] ArgoCD Application example that references the published Helm repo

There is no examples/ directory today (the only non-default values file is the e2e test fixture). The chart's zero-config design makes the first one nearly trivial: keycloak.hostname is the single required field.

Acceptance criteria:

• examples/nebari-values.yaml deploying on a NIC cluster with only a hostname edit
• examples/standalone-values.yaml if #122 lands on standalone-supported: yes
• examples/argocd-application.yaml pointing at the published Helm repo (coordinate with the helm-repository migration issue)

Part of #120.