Revamp CI/CD to use the new metaschema-xslt, generate artifacts on release

[nikitawootten-nist]

User Story

As an OSCAL user, I want an official source for downloading generated artifacts, such as schemas and converters, so that I can consume OSCAL artifacts within my project post-#1824.

More concretely, up until #1824 the existing CI/CD infrastructure was designed to commit to release branches automatically. This automatic committing creates a significant burden on release. A decision has been made to remove these generated artifacts from the OSCAL repository entirely.

Goals

• Streamline the schema and converter generation to be as simple to invoke as possible (another makefile?)
• Modify the existing action to build the schemas and converters without committing for PRs
• Also make the action upload schema artifacts on release

Dependencies

Following the #1824 merge and possibly a subset of usnistgov/metaschema-xslt#41

Acceptance Criteria

• The metaschema submodule is replaced with metaschema-xslt. All CI/CD infrastructure is overhauled to use the new XProc pipelines or helper script (see Helper script for all capabilities metaschema-xslt#41).
• A GHA action exists that generates all artifacts. The action should run on PRs and on a new release
• All OSCAL website and readme documentation affected by the changes in this issue have been updated. Changes to the OSCAL website can be made in the docs/content directory of your branch.
• A Pull Request (PR) is submitted that fully addresses the goals of this User Story. This issue is referenced in the PR.
• The CI-CD build process runs without any reported errors on the PR. This can be confirmed by reviewing that all checks have passed in the PR.

(For reviewers: The wiki has guidance on code review and overall issue review for completeness.)

Revisions

• This issue originates from this comment of Move web content from OSCAL, and other streamlining. #1824

[wendellpiez]

I am guessing the easiest change to make this work would be to adjust the CI/CD so that XSLTs currently at the top level are called instead in the schema-gen and converter-gen folders, where they have been relocated (and tested).

A more extensive change would entail working XProc like the docs pipelines, but that is arguably not necessary at this time (while it promises speed improvements among others).