Fixed Doxygit.cmake for 'make doxygit', added CommonDocumentation.cmake.

Argh you're right... I hadn't realized that. And I can't see how to make this portable. Unless you have a suggestion(?), I propose that we remove the doxygit target from here. doxycopy is sufficient IMO because users still need to go to the doc folder to review the changes, commit and push. So they can run doxygit in the doc folder as well. It clarifies the operations and removes multiple calls to doxygit when building multiple projects.