meson.build/docs: add a target to assemble a deb package

So, I find it sad people keep installing libinput with ninja install, and there's no proper way written anywhere on the internet. This is particularly bad on Ubuntu, because they package ancient libinput versions, and from my experience of looking at askubuntu questions it is a common pattern that someone stumbles upon a problem that have been solved in later libinput versions, but then there's no way to properly install latest libinput for them.

Recently at my dayjob I've been working with some Ubuntu packaging, and then I remember this situation, and decided to contribute a target for creating a deb package. Hopefully, this will fill the gap.