[APM] Document ILM policies and how to override them

[bmorelli25]

• Different ILM policies are set up for RUM and non-RUM transactions. Some details are in RUM: Route transaction|span events on agent.name apm-server#6480 and Add content to APM User guide apm-server#6378 (comment).
• Document these (for now) here: https://www.elastic.co/guide/en/apm/guide/current/ilm-how-to.html
• Adapt the "customize ILM" example I wrote for Elastic Agent to APM: https://www.elastic.co/guide/en/fleet/7.16/data-streams.html#data-streams-index-templates