XSLT M4 implementation repo cleanup

[wendellpiez]

Committer Notes

Addressing #164, removes lots of clutter, reorganizes, and adds readme files to help users and maintainers of the XSLT M4 implementation of Metaschema, going forward.

There are no changes here that should break anything. But a metaschema set (specifically: the OSCAL set) should be tested over this to ensure no bugs were introduced.

This commit also addresses, in part, usnistgov/OSCAL#966

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? tbd

Changes to Core Features:

See #164

• 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 website](https://pages.nist.gov/metaschema) and readme documentation affected by the changes you made? Changes to the website can be made in the website/content directory of your branch.

[david-waltermire]

Thanks!