Simplify and enhance CMake file in doc/

[tastytea]

This PR contains 3 changes:

• Use CMAKE_INSTALL_DOCDIR instead of share/doc/Tweeny – The result stays the same by default but package maintainers have it easier.
• Don't check TWEENY_BUILD_DOCUMENTATION – The sub-directory is only added if TWEENY_BUILD_DOCUMENTATION is true.
• Require Doxygen – When documentation is explicitly requested, it shouldn't silently fail.