Replace understanding docs with macros

SIDEBAR.ftl.md

@@ -28,6 +28,6 @@ There's more to building RESTful web services than is covered here. You can cont
 * [JSON][u-json]
 * [View Templates][u-view-templates]
 
-[u-rest]: /understanding/REST
-[u-json]: /understanding/JSON
-[u-view-templates]: /understanding/view-templates
+<@u_rest/>
+<@u_json/>
+<@u_view_templates/>

SIDEBAR.md

@@ -0,0 +1,33 @@
+## Related Resources
+
+There's more to building RESTful web services than is covered here. You can continue your exploration of Spring and REST with the following resources.
+
+### Getting Started Guides
+
+* [Consuming a RESTful Web Service][gs-consuming-rest]
+* [Building a Hypermedia-Driven REST Web Service][gs-rest-hateoas]
+* [Consuming REST Services with Spring for Android][gs-consuming-rest-android]
+* [Consuming XML from a REST Web Service with Spring for Android][gs-consuming-rest-xml-android]
+* [Building a RESTful Web Service with Spring Bootstrap Actuator][gs-actuator-service]
+
+[gs-consuming-rest]: /guides/gs/consuming-rest/
+[gs-consuming-rest-android]: /guides/gs/consuming-rest-android/
+[gs-rest-hateoas]: /guides/gs/rest-hateoas/
+[gs-consuming-rest-xml-android]: /guides/gs/consuming-rest-xml-android/
+[gs-actuator-service]: /guides/gs/actuator-service/
+
+### 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]
+* [View Templates][u-view-templates]
+
+[u-rest]: /understanding/REST
+[u-json]: /understanding/JSON
+[u-view-templates]: /understanding/view-templates