Replace understanding docs with macros

SIDEBAR.ftl.md

@@ -31,5 +31,5 @@ Spring Boot provides a wide range of support for various services, including mos
 * [REST][u-rest]
 * [JSON][u-json]
 
-[u-rest]: /understanding/REST
-[u-json]: /understanding/JSON
+<@u_rest/>
+<@u_json/>

SIDEBAR.md

@@ -0,0 +1,35 @@
+## Related resources
+
+Spring Boot provides a wide range of support for various services, including most services doumented in the Spring Getting Started guides. Some of these guides are listed here.
+
+### Getting Started guides
+
+* [Building a RESTful Web Service with Spring Boot Actuator][gs-actuator-service]
+* [Accessing Data with JPA][gs-accessing-data-jpa]
+* [Creating Asynchronous Methods][gs-async-method]
+* [Creating a Batch Service][gs-batch-processing]
+* [Securing a Web Application][gs-securing-web]
+* [Uploading a File][gs-uploading-files]
+* [Converting a Spring Boot JAR-based application to a WAR][gs-convert-jar-to-war]
+
+[gs-actuator-service]: /guides/gs/actuator-service
+[gs-accessing-data-jpa]: /guides/gs/accessing-data-jpa
+[gs-async-method]: /guides/gs/async-method
+[gs-batch-processing]: /guides/gs/batch-processing
+[gs-securing-web]: /guides/gs/securing-web
+[gs-uploading-files]: /guides/gs/uploading-files
+[gs-convert-jar-to-war]: /guides/gs/convert-jar-to-war
+
+### Tutorials
+
+* [Designing and Implementing RESTful Web Services with Spring][tut-rest]
+
+[tut-rest]: /guides/tutorials/rest
+
+### Concepts and technologies
+
+* [REST][u-rest]
+* [JSON][u-json]
+
+[u-rest]: /understanding/REST
+[u-json]: /understanding/JSON