setup the unsupported directory structure.

The unsupported module documentation is automatically generated in:
  build/doc/unsupported/
with bidirectional cross references.
I leave a class Foo in AdolcForward module to illustrate the
cross-reference behavior. I will remove it in the next commit.

unsupported/Eigen/AdolcForward

@@ -79,6 +79,16 @@ namespace unsupported {
   */
   //@{
 
+  /** \class Foo
+    * \brief a foo class
+    */
+  class Foo : public MatrixBase<Foo> {
+    public:
+      /** a member
+        * \see MatrixBase::sum() */
+      void member() {}
+  };
+
 } // namespace unsupported
 
 } // namespace Eigen