shape.adoc: put custom shapes in a new section and link to examples

docs/en/modules/ROOT/pages/shape.adoc

@@ -4,6 +4,7 @@
 :page-pagination:
 :url-api: https://stephengold.github.io/Libbulletjme/javadoc/master/com/jme3/bullet
 :url-enwiki: https://en.wikipedia.org/wiki
+:url-examples-shapes: https://github.com/stephengold/LbjExamples/tree/master/apps/src/main/java/com/github/stephengold/shapes
 :url-shape: https://stephengold.github.io/Libbulletjme/javadoc/master/com/jme3/bullet/collision/shapes
 :url-tutorial: https://github.com/stephengold/LbjExamples/blob/master/apps/src/main/java/com/github/stephengold/lbjexamples/apps
 
@@ -183,8 +184,10 @@ Similarly, collisions between `GImpactCollisionShape` and `PlaneCollisionShape`
 objects are never detected.
 ====
 
-Furthermore, you can develop custom collision shapes in Java by extending the
-abstract {url-shape}/CustomConvexShape.html[`CustomConvexShape`] class.
+== Custom shape classes
+
+Furthermore, you can define new shape classes by extending the
+abstract {url-shape}/CustomConvexShape.html[`CustomConvexShape`] class:
 
 [cols="2*",grid="none"]
 |===
@@ -194,11 +197,14 @@ abstract {url-shape}/CustomConvexShape.html[`CustomConvexShape`] class.
 {item} convex +
 {item} positive margin +
 {item} imprecise +
-{item} subclass determines scalability
+{item} each subclass determines its scalability
 |image:CustomConvex.png[height=160]
 
 |===
 
+Examples of this are found in
+{url-examples-shapes}/custom[the `com.github.stephengold.shapes.custom` package].
+
 == Convex versus concave
 
 The _convex_ collision-shape classes are those