Add Envoy OTLP dashboard and docs

[keremercin]

Summary

• add an Envoy monitoring dashboard that uses OTLP metrics for general, request/response, latency, error, resource and network views
• wire namespace, deployment.environment, service.name and cluster variables into every panel so the dashboard is reusable across environments
• document the ingestion prerequisites for Envoy’s OpenTelemetry stat sink and the dashboard variables

Testing

• python -m json.tool envoy/envoy-otlp-v1.json

Closes SigNoz/signoz#6020
/claim #6020