doc: add a full "tutorial" on how to get a device added

This was (partially) written while working on !158 with the intention of having some documentation that really guides a completely new developer on how to add support for a device. The K20 is the one I had on hand and need adding - it's a remote without a pen so some bits are easier or missing that other devices would need.

Still, I think this is a useful addition to have because it's a lot easier to point to it than answering the same question in multiple PRs.

At this stage it's still missing the bits for overwriting the firmware mode, just like !158.