Clarify link semantics in SSP model

[Rene2mt]

Committer Notes

This pull request adds content to the SSP concepts page to clarify link semantics in the SSP model to address issue #1062.

All Submissions:

• Have you selected the correct base branch per Contributing guidance?
• 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]
• Do all automated CI/CD checks pass?

Changes to Core Features:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your core changes, as applicable?
• Have you included examples of how to use your new feature(s)?
• Have you updated all OSCAL website and readme documentation affected by the changes you made? Changes to the OSCAL website can be made in the docs/content directory of your branch.

[Rene2mt]

@david-waltermire-nist I meant to PR from my Rene2mt:1062-ssp-link-semantics branch, not my main branch. I'm closing this PR and will submit a new one.