OSCAL SSP: OSCAL SSP: Separate and revise required attachments content

[brian-ruf]

User Story

As an implementer of tools for authoring of a FedRAMP SSP content, I need to understand the correct way to express the FedRAMP SSP required attachments details in OSCAL

Content Location

This content must reside in the Developer Hub as follows:

Repo Location:

/content/documentation/ssp/oscal-representation/required-attachments.md

URL Location:

https://automate.fedramp.gov/documentation/ssp/oscal-representation/required-attachments/

Revision Work Alignment

This content must align with the work performed in issue #809 :