Feature markdown guides

[david-waltermire]

Committer Notes

This PR represents a work-in-progress to represent the FedRAMP OSCAL guides in Markdown for publishing as HTML instead of the current PDF format.

This feature branch will be used to refine the Markdown guide revisions prior to initially posting them as HTML.

All Submissions:

• [ ] Have you selected the correct base branch per Contributing guidance?
• [ ] Have you set "Allow edits and access to secrets by maintainers"?
• [ ] Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• [ ] Have you squashed any non-relevant commits and commit messages? [instructions]
• [ ] Have you added an explanation of what your changes do and why you'd like us to include them?
• [ ] If applicable, have all FedRAMP Documents Related to OSCAL Adoption affected by the changes in this issue have been updated.?
• [ ] If applicable, does this PR reference the issue it addresses and explain how it addresses the issue?

By submitting a pull request, you are agreeing to provide this contribution under the CC0 1.0 Universal public domain dedication.