[Serverless]: Add migration guideline for agents

[ppf2]

Serverless Docs

Elastic Observability

Description

This applies to both o11y and security.

While we wait for migration tools to be available on Serverless, can we add a section on how users can migrate their Agents to Serverless? We have a stop gap article in support (https://support.elastic.dev/knowledge/view/64804a87) but it will be better if we can have such information directly in documentation.

Resources and additional context

https://support.elastic.dev/knowledge/view/64804a87

[ppf2]

Closing in favor of https://github.com/elastic/docs-content-internal/issues/209