Revise trestle project structure & documentation to clarify public vs private signatures.

[butler54]

Issue description / feature objectives

Currently trestle has most methods scoped publicly. This creates uncertainty around which methods are acceptable to change w/o breaking backwards compatibility.

Identify all critical public api signatures

• OscalBaseModel
• load distributed
• repository api
• CLI signatures
• certain drawio and markdown apis and IO
• load_distributed
• trestle.oscal.*

Document these then:

• Refactor exisiting code to 'hide' imports as appropriate from public consumptions.

[jpower432]

@butler54 This could help create some boundaries for this proposal - oscal-compass/community#68. Should we close or keep this open?