This PR provides more conventional settings for building and installing the Doxygen generated HTML documentation. The HTML documentation is now generated under a separate html directory instead of raw ${CMAKE_CURRENT_BINARY_DIR}.

Besides this approach being standard practice, it has these additional benefits:

1. No tempering by CMake specific cruft. Hence the removal of the CMakeFiles exclusion pattern,
2. No conflicts with other potential docs (manpage, latex, pdf...). Hence the removal of the man exclusion pattern.
3. The HTML documentation is directly installed under ${AF_INSTALL_DOC_DIR}/html which is the expected install path for HTML related content.

In addition, the COMMAND cmake -E remove ${CMAKE_CURRENT_BINARY_DIR}/.git is removed due to the fact that the .git folder in assets may have already been stripped from the release tarball, as it is the policy in Linux distributions. Instead an exclusion pattern ".git" is used on the corresponding installation target.