[Community]: Clarify Agent output configuration for sending to a Gateway collector

### What documentation page or section is affected

Docs page: https://www.elastic.co/docs/reference/edot-collector/config/default-config-standalone#agent-mode

### Additional info

Context: raised from a discussion in the otel Slack channel — requester noted the page announces Agent mode but lacks an example of Agent output configuration for Gateway collectors. Managed OTLP endpoint examples exist for Serverless/ECH but not for self-managed.

### What should change?

Add clarification and an example for configuring the Agent to send data to a Gateway collector. The current docs mention Agent mode but do not show the output configuration for sending to a Gateway collector. Please add:

- A short explanation that Agent can forward to a Gateway collector and when that is appropriate.
- A concrete output configuration example/snippet for the Agent that targets a Gateway collector (OTLP endpoint), similar to the Managed OTLP endpoint example used for Serverless/ECH, but applicable for self-managed Gateway collectors.
- Notes about differences or limitations compared to the Managed OTLP endpoint (Managed OTLP is only available for Serverless and ECH).

[View original Slack conversation](https://elastic.slack.com/archives/C0JF80CJZ/p1783369040011689?thread_ts=1783369040.011689&cid=C0JF80CJZ)