[BUG]no method to install docs

[bvanevery]

CMakeLists.txt defines CEGUI_DOC_INSTALL_DIR, even with a nice CEGUI version number. But nobody uses it, "make install" doesn't install docs, and there's no "make" target for installing the doxygen-generated html docs. I don't see why people would generally want to leave the docs in a CMake out-of-source build directory.