Develop/write a general design document for the communication bus work

[gigapod]

Place in a docs directory, written in markdown.

Goal - to evolve this as the project evolves -- and then place this in the final documentation for this project (GitHub Pages).

[gigapod]

This is mostly done - enough to close this. https://github.com/sparkfun/SparkFun_Toolkit/blob/main/docs/ar_ibus.md