Exclude doxygen docs from documentation install step

We might want to keep them in at some point, but currently the
doxygen docs are very incomplete and unfinished.

CMakeLists.txt

@@ -134,6 +134,7 @@ if (NOT_SUBPROJECT)
           docs/
         DESTINATION
           "${CMAKE_INSTALL_DOCDIR}"
+        PATTERN "doxygen" EXCLUDE
       )
     endif()