Example documentation moved to xmltest.cpp

Managed to move documentation for examples to their source file. In this case, documentation block has to come **after** the code it documents, otherwise `@skip` and `@until` commands match themselves. Why Doxygen doesn't skip doc blocks while matching these, I don't know.

I also upgraded *dox* to latest Doxyfile format, which resulted in some deprecated options being removed, and set readme.md to be used for documentation's MainPage.