Update data types specifications in Pages site

[wendellpiez]

User Story:

Atomic data types (for field and flag values) must be documented for Metaschema.

These can be back ported and updated from the OSCAL Pages site.

This helps address Issue #224.

Data type descriptions should be normative and should address how datatypes can (and cannot) be validated lexically to ensure values can be processed without (parsing) errors.

Goals:

Update the normative specs for Metaschema datatypes. The place to start (and maybe finish) is at https://github.com/usnistgov/metaschema/blob/main/website/content/specification/syntax/_index.md

Further refactoring of the site is not a goal, but these specs must be as accurate and complete as possible.

Dependencies:

Stability wrt #224.

Acceptance Criteria

• All website and readme documentation affected by the changes in this issue have been updated. Changes to the 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.