[docs][APM] Add APM settings page (#53172) (#53205)

docs/apm/settings.asciidoc

@@ -0,0 +1,10 @@
+// Do not link directly to this page.
+// Link to the anchor in `/docs/settings/apm-settings.asciidoc` instead.
+[[apm-settings-in-kibana]]
+=== APM settings in Kibana
+
+You do not need to configure any settings to use APM. It is enabled by default.
+If you'd like to change any of the default values,
+copy and paste the relevant settings below into your `kibana.yml` configuration file.
+
+include::./../settings/apm-settings.asciidoc[tag=general-apm-settings]

docs/apm/using-the-apm-ui.asciidoc

@@ -8,15 +8,18 @@ Not to worry, we've created this guide to help you get the most out of Elastic A
 
 APM is available via the navigation sidebar in {Kib}.
 
-* <<filters,Filters>>
-* <<services,Services overview>>
-* <<traces,Traces overview>>
-* <<transactions,Transaction overview>>
-* <<spans,Span timeline visualization>>
-* <<errors,Debug errors>>
-* <<metrics,Metrics overview>>
-* <<machine-learning-integration,Machine learning integration>>
-* <<query-bar,Advanced queries>>
+* <<filters>>
+* <<services>>
+* <<traces>>
+* <<transactions>>
+* <<spans>>
+* <<errors>>
+* <<metrics>>
+* <<machine-learning-integration>>
+* <<agent-configuration>>
+* <<advanced-queries>>
+* <<apm-settings-in-kibana>>
+* <<troubleshooting>>
 
 include::filters.asciidoc[]
 
@@ -36,4 +39,6 @@ include::agent-configuration.asciidoc[]
 
 include::advanced-queries.asciidoc[]
 
+include::settings.asciidoc[]
+
 include::troubleshooting.asciidoc[]

docs/settings/apm-settings.asciidoc

@@ -13,6 +13,10 @@ copy and paste the relevant settings below into your `kibana.yml` configuration
 [[general-apm-settings-kb]]
 ==== General APM settings
 
+// This content is reused in the APM app documentation.
+// Any changes made in this file will be seen there as well.
+// tag::general-apm-settings[]
+
 xpack.apm.enabled:: Set to `false` to disabled the APM plugin {kib}. Defaults to
 `true`.
 
@@ -37,3 +41,4 @@ apm_oss.metricsIndices:: Matcher for indices containing metric documents. Defaul
 
 apm_oss.sourcemapIndices:: Matcher for indices containing sourcemap documents. Defaults to `apm-*`.
 
+// end::general-apm-settings[]