Added link to core docs

doc/xdocs/navigation.xml

@@ -28,14 +28,18 @@
     </links>
 
     <menu name="Overview">
-      <item name="Home"                    href="index.html"/>
-      <item name="Reference"               href="reference.html"/>
-      <item name="Building"                href="building.html"/>
-      <item name="Download"                href="download.html"/>
+      <item name="Home" href="index.html"/>
+      <item name="Reference" href="reference.html"/>
+      <item name="Building" href="building.html"/>
+      <item name="Download" href="download.html"/>
+      <item name="Subprojects" href="projects-overview.html"/>
     </menu>
 
-    <menu name="Subprojects">
-      <item name="Overview" href="projects-overview.html"/>
+    <menu name="Core">
+      <item name="Core" href="multiproject/acegi-security/index.html"/>
+    </menu>
+
+    <menu name="Adapters">
       <item name="CAS" href="multiproject/acegi-security-cas/index.html"/>
       <item name="Catalina" href="multiproject/acegi-security-catalina/index.html"/>
       <item name="JBoss" href="multiproject/acegi-security-jboss/index.html"/>