Add table of contents to user guide

The user guide page can be directly linked from the site menu.

Added selected section links to the site menu.
diff --git a/src/site/apt/userguide/rng.apt b/src/site/apt/userguide/rng.apt
index 9476d58..d725750 100644
--- a/src/site/apt/userguide/rng.apt
+++ b/src/site/apt/userguide/rng.apt
@@ -15,9 +15,25 @@
 ~~ limitations under the License.
 ~~
 
-  -----------------------------
-  The Apache Commons RNG User Guide
-  -----------------------------
+The Apache Commons RNG User Guide
+
+* Table of contents
+
+  * {{{a1._Purpose}Purpose of the library}}
+
+  * {{{a2._Usage_overview}Usage overview (for users)}}
+
+  * {{{a3._Library_layout}Library layout (for developers)}}
+
+  * {{{a4._Performance}Performance}}
+
+  * {{{a5._Quality}Quality}}
+
+  * {{{a6._Examples}Examples}}
+
+  * {{{a7._Release_compatibility}Release compatibility}}
+
+  * {{{a8._Dependencies}Dependencies}}
 
 1. Purpose
 
diff --git a/src/site/site.xml b/src/site/site.xml
index 156309e..dda3202 100644
--- a/src/site/site.xml
+++ b/src/site/site.xml
@@ -45,7 +45,10 @@
     </menu>
 
     <menu name="User Guide">
-      <item name="Contents" href="/userguide/index.html"/>
+      <item name="Contents" href="/userguide/rng.html#Table_of_contents"/>
+      <item name="Usage" href="/userguide/rng.html#a2._Usage_overview"/>
+      <item name="Performance" href="/userguide/rng.html#a4._Performance"/>
+      <item name="Quality" href="/userguide/rng.html#a5._Quality"/>
     </menu>
   </body>