Add New Relic to monitoring section

[lukekim]

[claudespice]

Noting current state for planning:

New Relic integration already works today via two paths that are documented:

• Prometheus scrape: Prometheus Metrics Endpoint — New Relic is listed as a compatible target alongside Datadog and Chronosphere.
• OpenTelemetry push: OpenTelemetry Metrics Exporter — New Relic is listed as a supported OTLP backend.

What's missing (and what this issue likely intends) is a dedicated monitoring/new-relic walkthrough analogous to the existing monitoring/datadog and monitoring/grafana pages, with a concrete Spicepod snippet showing the New Relic OTLP endpoint + api-key header configuration.

Leaving open for someone who can verify the walkthrough against a real New Relic account.