Update Documentation -> POA&M section

[Rene2mt]

As a developer, I want to update the Documentation - POA&M section and subsections so that stakeholders understand how to express a FedRAMP POA&M in OSCAL.

Todo:

• [x] Rename section and update menu
• [x] Remove references to "this document" or "this guide"
• [x] Update overview sub-section, removing any duplicate content
• [x] Remove section numbers from headers
• [x] Fix / update diagrams
• [x] Fix / update screenshots
• [ ] Fix / update code snippets
• [ ] Apply consistent styling & theming
• [ ] Editorial review