meson.build: disable documentation by default

meson.build: disable documentation by default

This does little other than drag in a whole bunch of dependencies. The
libinput documentation is designed to be consumed online, so there's no
need building it on every machine.

We leave the dependencies installed in the images because it's a lot
easier to remove them and test if the build still works than adding them
and dragging in every updated package since we built the image.

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