Add precise docs on responsible-party and role in SSP

[aj-stein-gsa]

The documentation site provides high-level description of the role assembly and its use (see https://automate.fedramp.gov/documentation/general-concepts/oscal-metadata/#working-with-role-assemblies) but is not explicit about the need for a corresponding responsible-party). May need to update documentation to make this rule clearer.

This section of the documentation (https://automate.fedramp.gov/documentation/ssp/4-ssp-template-to-oscal-mapping/) calls out specifict roles (e.g., "system-owner", "authorizing-official", etc.), and does mention the use of responsible-party with provided examples.

Documentation needs to be added for the role-defined-authorizing-official-poc constraint.

See original context for this work in GSA/fedramp-automation#754 (review).