feat: Add Istio configuration example

[as51340]

Adds examples/gateway/istio-values.yaml — an Istio counterpart to the Traefik Gateway API example (#271).

The Memgraph HA chart exposes Bolt through TCPRoute, which is an alpha Gateway API resource that Istio gates behind PILOT_ENABLE_ALPHA_GATEWAY_API=true. Without the flag the failure is easy to misread: the Gateway shows Programmed=True and its LoadBalancer Service lists all the ports, but the TCPRoutes get no status and every connection times out. The example provides istiod values with the flag set, plus the full install sequence (experimental Gateway API CRDs → istio-base → istiod → HA chart with gatewayClassName=istio), the native-sidecars note for the cluster-setup Job, and the neo4j:// routing / external-dns bolt_server note.

Verified end to end on an AKS cluster: TCPRoutes Accepted/ResolvedRefs and Bolt handshakes complete through the Istio-provisioned gateway on 7687 (coordinators, single shared listener) and 9000/9001 (data instances).

[as51340]

Tracking

• [Link to Epic/Issue]

Standard development

• Update / add chart templates, values, and NOTES.txt as needed (example file only, no chart changes)
• Run helm lint and helm template locally against the changed chart(s) (no chart changes)
• Verify the chart installs and upgrades cleanly on a test cluster (verified on AKS with Istio)

Labels checklist

• Add a docs label (exactly one): docs-needed
• Add at least one component label: memgraph-ha
• Add at least one type label: feature
• Assign the PR to a milestone

Documentation checklist

• Write a release note, including added/changed clauses
• [ Documentation PR link memgraph/documentation#XXXX ]
  • Is back linked to this development PR

[as51340]

Documentation PR: memgraph/documentation#1741