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

This was (partially) written while working on !158 (merged) 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 (merged).