docs: Fix service name from mimir-nginx to mimir-gateway in helm charts guide

[shouhei]

What this PR does

Fix outdated service name in the Helm charts getting started guide.

• Change mimir-nginx to mimir-gateway in get-started-helm-charts/_index.md
• The service name was updated in helm: remove nginx from helm chart #12702 but the documentation wasn't fully updated

---

Note

Replaces references to mimir-nginx with mimir-gateway in pod list, Alloy remote_write URL, and Grafana data source URL.

• Docs (Helm guide):
  • Replace mimir-nginx with mimir-gateway:
    • Example pod list entry updated to mimir-gateway.
    • Alloy config prometheus.remote_write endpoint switched to http://mimir-gateway.mimir-test.svc.cluster.local/api/v1/push.
    • Grafana Prometheus data source URL updated to http://mimir-gateway.mimir-test.svc:80/prometheus.

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[CLAassistant]

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