[Docs] Document setup instructions for CCS with APM

[nehaduggal]

APM supports CCS but don't have the setup documented. While we implement CCS configuration from the Kibana UI, update APM documentation to reflect CCS support and the setup instructions.

[bmorelli25]

This PR should also add documentation on runtime index config in Kibana. Details on each below.

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APM with CCS

• Before users can use ccs in apm ui indices they have to setup the remote cluster via this page
  <KIB_URL>/app/kibana#/management/elasticsearch/remote_clusters/add
• To configure CCS, all of the kibana.yml settings for indices (apm_oss.*Indices) should include *:apm-*,apm-*. APM UI CCS support kibana#39759
• Related: CCS on ESS blog post.

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Runtime index config

• Issue: Runtime APM UI index configuration kibana#39741
• Design: [APM] Implementation of runtime UI index configuration kibana#45247
• Implementation PR: [APM] Add UI Indices runtime configuration kibana#48079

[bmorelli25]

This issue also requires changes to the APM app set up process. Two PRs will be needed to close this issue:

• CCS documentation in the APM Overview: [docs] Adds APM app cross cluster search documentation apm-server#3089
• Runtime index config & APM app set up process changes: Still figuring this out. Can't get runtime index config to work right now [docs][APM] Add runtime index config documentation kibana#53907