[XAP] finalise XAP spec definition format

[tzarc]

XAP Task Info

Original Issue
Original spec document
Current XAP Definitions
Current XAP Generated Docs
Placeholder PR

Description

Any work in this area needs to be discussed with QMK Collaborators first, and in this case primarily with @tzarc due to their prior work in this area.

XAP protocol definitions under data/xap need to be finalised such that they contain documentation as well as enough information to generate firmware-side code and host app binding code. These are the "single source of truth" for everything related to the XAP protocol. All information required such as mappings or transformations needs to be defined in the XAP definitions, rather than "hacked about" in generator code.

An appropriate *.jsonschema is required.