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/doc/unsupported.dox

@@ -1,15 +0,0 @@
-
-namespace Eigen {
-
-namespace unsupported {
-
-/** \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.
-  */
-
-} // namespace unsupported
-
-} // namespace Eigen