[Cloud Security] Add Agentless docs to serverless CSPM documentation

[maxcold]

User story As a user I want to be able to find information about Agentless CSPM so that I learn how to use it

Motivation

As we added Agentless setup option in CSPM for all cloud providers, we need to have documentation covering it. As Agentless is only supported in serverless projects atm, probably a good place for such docs is https://docs.elastic.co/serverless

Definition of done

• [ ] Agentless CSPM documentation exists on one of the dcumentation resources

Out of scope

Related tasks/epics

Team tag

@elastic/kibana-cloud-security-posture