Add troubleshooting crosslink (#4218)

Author: theletterf

docs/reference/index.md

@@ -33,3 +33,6 @@ More detailed information on how the Agent works can be found in the [FAQ](/refe
 
 APM Agents work in conjunction with the [APM Server](docs-content://solutions/observability/apm/index.md), [Elasticsearch](docs-content://get-started/index.md), and [Kibana](docs-content://get-started/the-stack.md). The [APM Guide](docs-content://solutions/observability/apm/index.md) provides details on how these components work together, and provides a matrix outlining [Agent and Server compatibility](docs-content://solutions/observability/apm/apm-agent-compatibility.md).
 
+## Troubleshooting
+
+If you're experiencing issues with the APM Java agent, refer to [Troubleshooting](docs-content://troubleshoot/observability/apm-agent-java/apm-java-agent.md).

docs/reference/toc.yml

@@ -43,4 +43,6 @@ toc:
   - file: overhead-performance-tuning.md
   - file: frequently-asked-questions.md
   - file: community-plugins.md
-  - file: upgrading.md
+  - file: upgrading.md
+  - title: Troubleshooting
+    crosslink: docs-content://troubleshoot/observability/apm-agent-java/apm-java-agent.md