meson: disable documentation by default

libwacom's documentation is based on doxygen and thus designed to be
consumed online - we now have this in github pages. So let's disable the
documentation build by default.

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

Author: whot

meson_options.txt

@@ -1,6 +1,6 @@
 option('documentation',
        type: 'feature',
-       value: 'auto',
+       value: 'disabled',
        description: 'Build doxygen documentation [default=auto]')
 option('udev-dir',
        type: 'string',