[APM] Uses doc link service instead of ElasticDocsLink for linking custom links (#111149)

Author: szabosteve

docs/development/core/public/kibana-plugin-core-public.doclinksstart.links.md

@@ -12,6 +12,7 @@ readonly links: {
         readonly apm: {
             readonly kibanaSettings: string;
             readonly supportedServiceMaps: string;
+            readonly customLinks: string;
             readonly droppedTransactionSpans: string;
             readonly upgrading: string;
             readonly metaData: string;