Add documentation aimed at trestle SDK user

[fsuits]

Issue description / feature objectives

Add documentation describing how to use the trestle sdk, including the overall design of the codebase and how the OSCAL classes are laid out.

Caveats / Assumptions

Keep it distinct from what users see. Link it to new and existing demos.

Completion Criteria

Have new documentation that gets sdk users up to speed and describes how things like the OSCAL class files came about.