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.

doc/unsupported_modules.dox

@@ -0,0 +1,16 @@
+
+namespace Eigen {
+
+/** \defgroup Unsupported_modules Unsupported modules
+  *
+  * The unsupported modules are contributions from various users. They are
+  * provided "as is", without any support. Nevertheless, they are subject to be
+  * included in Eigen in the future.
+  */
+
+// please list here all unsupported modules
+
+/** \ingroup Unsupported_modules
+  * \defgroup AdolcForward_Module Adolc forward module */
+
+} // namespace Eigen