[Website]: Improvements to the docs regarding ES OTLP Endpoint

### Type of issue

None

### What documentation page is affected

https://www.elastic.co/docs/manage-data/data-store/data-streams/tsds-ingest-otlp

### What happened?

## Suggestion (1)

> ECE, ECK, and self-managed: Use the Elasticsearch OTLP endpoint described on this page, ideally through an OpenTelemetry Collector in [Gateway mode](https://www.elastic.co/docs/reference/edot-collector/config/default-config-standalone#gateway-mode).

I would refrain to put this into the docs, at least until EDOT GW recommended configs will be updated to leverage it.

Also that warning might allude to the fact this would support any signal, while this is specific to metrics.

## Suggestion (2)

Update the header to reflect since which version is Tech Preview and which version is GA.

### Additional info

_No response_