Document how to proxy/load balance upstream OTel Collector requests to the EDOT Collector

The existing [Proxy requests to APM Server](https://www.elastic.co/docs/solutions/observability/apm/opentelemetry/upstream-opentelemetry-collectors-language-sdks#apm-open-telemetry-proxy-apm) section in the upstream OTel Collector guide covers how to proxy OTLP requests to APM Server.

Now that the recommended target for self-managed deployments is the [EDOT Collector as a gateway](https://www.elastic.co/docs/reference/edot-collector/modes) rather than APM Server directly, there's no equivalent guidance for proxying to the EDOT Collector.

A new page or section could cover:

* L4 vs L7 load balancer trade-offs when fronting the EDOT Collector gateway (OTLP ports 4317/4318)
* HTTP/2 and gRPC considerations (same constraints as APM Server apply)
* Some examples: AWS ALB, NGINX Ingress Controller on Kubernetes

This content belongs in the EDOT Collector docs rather than the APM OpenTelemetry section.

Start by reviewing the existing “Proxy requests to APM Server” section in the upstream OTel Collector guide and the EDOT Collector modes page. Add EDOT gateway guidance covering L4/L7 trade-offs, OTLP ports 4317/4318, HTTP/2 and gRPC constraints, and AWS ALB and NGINX Ingress Controller examples; done means the guidance is published in the EDOT Collector documentation.

Written by the indexing model from the issue text.