docs: improve getting started experience (elastic#4222)

Author: bmorelli25

guide/apm-breaking-changes.asciidoc

@@ -19,7 +19,7 @@ This section discusses the changes that you need to be aware of when migrating y
 * <<breaking-6.5.0>>
 * <<breaking-6.4.0>>
 
-Also see <<apm-release-notes>>.
+Also see <<whats-new>>.
 
 //NOTE: The notable-breaking-changes tagged regions are re-used in the
 //Installation and Upgrade Guide

guide/apm-doc-directory.asciidoc

@@ -1,12 +1,12 @@
 [[components]]
-== Components and documentation
+=== Components and documentation
 
-Elastic APM consists of four components: *APM Agents*, *APM Server*, *Elasticsearch*, and *Kibana*.
+Elastic APM consists of four components: *APM agents*, *APM Server*, *Elasticsearch*, and *Kibana*.
 
 image::images/apm-architecture-cloud.png[Architecture of Elastic APM]
 
 [float]
-=== APM Agents
+==== APM Agents
 
 APM agents are open source libraries written in the same language as your service.
 You may only need one, or you might use all of them.
@@ -20,12 +20,14 @@ Each agent has its own documentation:
 * {apm-java-ref-v}/intro.html[Java agent]
 * {apm-dotnet-ref-v}/intro.html[.NET agent]
 * {apm-node-ref-v}/intro.html[Node.js agent]
+// Uncomment PHP Agent when ready
+// * {apm-php-ref}/intro.html[PHP agent]
 * {apm-py-ref-v}/getting-started.html[Python agent]
 * {apm-ruby-ref-v}/introduction.html[Ruby agent]
 * {apm-rum-ref-v}/intro.html[JavaScript Real User Monitoring (RUM) agent]
 
 [float]
-=== APM Server
+==== APM Server
 
 APM Server is an open source application that receives performance data from your APM agents.
 It's a {apm-server-ref-v}/overview.html#why-separate-component[separate component by design],
@@ -34,7 +36,7 @@ which helps keep the agents light, prevents certain security risks, and improves
 After the APM Server has validated and processed events from the APM agents,
 the server transforms the data into Elasticsearch documents and stores them in corresponding
 {apm-server-ref-v}/exploring-es-data.html[Elasticsearch indices].
-In a matter of seconds you can start viewing your application performance data in the Kibana APM app.
+In a matter of seconds, you can start viewing your application performance data in the Kibana APM app.
 
 The {apm-server-ref-v}/index.html[APM Server reference] provides everything you need when it comes to working with the server.
 Here you can learn more about {apm-server-ref-v}/getting-started-apm-server.html[installation],
@@ -43,14 +45,14 @@ Here you can learn more about {apm-server-ref-v}/getting-started-apm-server.html
 {apm-server-ref-v}/monitoring.html[monitoring], and more.
 
 [float]
-=== Elasticsearch
+==== Elasticsearch
 
 {ref}/index.html[Elasticsearch] is a highly scalable open source full-text search and analytics engine.
 It allows you to store, search, and analyze large volumes of data quickly and in near real time.
 Elasticsearch is used to store APM performance metrics and make use of its aggregations.
 
 [float]
-=== Kibana APM app
+==== Kibana APM app
 
 {kibana-ref}/index.html[Kibana] is an open source analytics and visualization platform designed to work with Elasticsearch.
 You use Kibana to search, view, and interact with data stored in Elasticsearch.
@@ -63,4 +65,6 @@ The following sections will help you get started:
 * {apm-app-ref}/apm-getting-started.html[Get started]
 * {apm-app-ref}/apm-how-to.html[How-to guides]
 
-APM also has built-in integrations with Machine Learning. To learn more about this feature, refer to the Kibana documentation for {kibana-ref}/machine-learning-integration.html[Machine learning integration].
+APM also has built-in integrations with Machine learning. To learn more about this feature,
+or the anomaly detection feature that's built on top of it,
+refer to {kibana-ref}/machine-learning-integration.html[Machine learning integration].