Add crosslinks in APM agents to Troubleshooting docs

APM reference documentation lacks links to APM agents troubleshooting docs, such as https://www.elastic.co/docs/troubleshoot/observability/apm-agent-java/apm-java-agent for example.

To improve findability, we're going to add crosslinks to the APM agents reference navs pointing to the respective APM agent troubleshooting doc, replicating the experience of the previous docs.