Doc: Adding C++ classes page to list of modules.

-this page will then be listed in the "All Classes by Module" page.

Change-Id: I7804d9aa780d28224edfcb310a6e2d362d329faa
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>