heater/shaker: stand up integration documentation

[sfoster1]

There should be some documentation around what gcodes are respected and their formats, and what error codes exist and their formats. To be most useful this should be maintained along with the code.

[sfoster1]

Done (temporarily) not maintained by the code in https://opentrons.atlassian.net/wiki/spaces/RPDO/pages/512819201/Interaction+GCodes+and+Errors, will be publicized later