Update Documentation -> SSP Section

[Rene2mt]

As a developer, I want to update the Documentation - SSP section and subsections so that stakeholders understand how to express a FedRAMP SSP 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
• [x] Fix / update code snippets
• [x] Apply consistent styling & theming
• [x] Editorial review

[Rene2mt]

Editorial review completed. Site was updated to address identified issues. Issue resolved with merged PR #20 , #24, #30, #32 and #34.