343: Add new sensor documentation

[truth-quark]

This documentation PR resolves #343. I've added @jeremyh as an optional reviewer if he's got spare time.

The first commit is the bulk of the documentation of the sensor addition process. I think I've got the required details included based on a review of the TSX sensor PR.

Further required changes include:

• TODOs referencing docs not yet in the gamma_workflow branch
• Troubleshooting... requires a reminder of ard.py breakages from Mitch :-)

I'll add these changes after a review and when other docs PRs are merged.

[truth-quark]

The last set of changes is pretty minor.