Data lifecycle docs: Data retention process for Serverless

[kilfoyle]

From the feedback on ILM docs session, I'm not sure to what extent this is already documented, so this may just be a matter of confirming that the procedure is already clear in the docs.

Serverless doesn't have ILM; uses data streams. Typical steps are:

• Add System integration.
• Fill in fields.
• Install agent.
• Agent creates the data stream.
• Data stream uses index template.
• Index template has the “Data retention” enable setting.

[kilfoyle]

I think this can be closed now, since:

• We've added an ILM Availability section to guide Serverless users to use data stream lifecycle.
• The process is outlined in Configuring data stream lifecycle
• Though it's specific to ILM, to better address the case of people using the Systems Integration or other integrations to find the underlying data stream, we now have Manage the lifecycle policy for integrations data.