doc/api: fix the doxygen output handling

doc/api: fix the doxygen output handling

Using output: ['.'] broke ninja after ninja clean - it removed the whole
directory and thus the meson-generated configure_files (i.e. all the
doxygen sources we copied). ninja didn't know how to build those.

Fix this by rearranging the doxygen output to build into a different
directory now and setting the output to that. This doesn't exactly *fix*
things since that directory is no longer removed during ninja clean, but at
least the build no longer fails.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>