Website updates

[brian-ruf]

Committer Notes

Primarily brings the catalog and profile model documentation into alignment from a presentation perspective (same information in the same place). Additional model documentation will be updated in later sprints.
Updated content links to point to the new oscal-content repository.

Also updates the layer documentation, which introduces all layers and models.

Finally, updated the downloads section of the website content to include a link for the MR3 download package.

All Submissions:

• Have you followed the guidelines in our Contributing document?
• 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.

[david-waltermire]

There are still concepts in the catalog layer documentation that need to be moved to the new concepts page. Can you make this change?

[david-waltermire]

@brianrufgsa This looks good. Can you rebase this against upstream master and mark the PR as ready (its currently draft)? Thanks!