All markdown files can be included into the OSCAL SSP model as appropriate

[rahearn]

Existing tooling does not break non-control content out into markdown files.

As a developer, I want to have tooling to write all descriptions in markdown files, so that I can avoid doing any JSON editing when putting together my SSPP.

Acceptance criteria:

• A markdown folder and document structure can be created to put content into
• That markdown can be automatically pulled back into the OSCAL SSP model in the appropriate fields.

[rahearn]

Potentially related: oscal-compass/compliance-trestle#1539

[rahearn]

wait until oscal-compass/compliance-trestle#1617 has been implemented