[Agent] New serverless config example for Logstash output 8.19

[karenzone]

Provides a Serverless config example, showing Elastic endpoint URL and an API key to establish the connection from Logstash to Serverless. Cloud ID is not viewable in the Cloud console.

Original issue: elastic/docs-content#5192
Parent issue: https://github.com/elastic/docs-content-internal/issues/777

Refactored backport of elastic/docs-content#5193

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⚠️ The sha of the head commit of this PR conflicts with #1892. Mergify cannot evaluate rules on this PR. ⚠️