[Serverless]: Add either "over HTTP" or "over gRPC" to the sub headings within the "APM OpenTelemetry intake" section

[Rossall82]

Serverless Docs

Observability Intake Serverless API

Description

Make it clearer which protocol the current set of sub headings are for:

• Send OTLP metrics
• Send OTLP traces
• Send OTLP logs

...by adding the protocol as a suffix. It would help reduce cognitive load when looking for the relevant example depending on which protocol is desired. Alternatively group them under a section per protocol.
The description states which protocol is used for the current API but I think it could be made clearer.

Resources and additional context

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[elasticmachine]

Pinging @elastic/obs-ux-infra_services-team (Team:obs-ux-infra_services)