OSCAL schema documentation revamp (main schema docs pages) for Metaschema M4.

[wendellpiez]

User Story:

With changes and new features in Metaschema (see #721) come new requirements for documentation.

Enhancements in two areas need to be reflected in usable documentation:

• Capability to write local definitions overriding global (general) definitions
• Capability to define constraints to be enforced with "precision targeting" i.e. nested and conditional constraints
  • Enumerated value testing, but also
  • Key/keyref (index) checking ("am I related to one of those?"); uniqueness requirements
  • Regex pattern matching
  • Cardinality checking (there must be at least one, two, no more than eight, 20, x)
  • Value comparison ("this should be greater than that other thing")

Splitting this further into Issues could make sense.

Goals:

• Integrate new production pipeline for generated schema documentation into Hugo
• Extend documentation (see Update schema documentation production to current Metaschema model metaschema-xslt#14):
  • Local definitions
  • use-name? (Hasn't been well tested yet: tbd.)
  • Constraints (various types, indexed to targets)
  • Various mop up / tbd

It might be necessary to triage these features and/or refactor design, to achieve usable results. (For example, break out constraints descriptions into a separate page.)

Dependencies:

#721, #724

Acceptance Criteria

• 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.

[wendellpiez]

Status 2-Sept-2020

Refactored this Issue to exclude the model maps, now in Issue #750.

[wendellpiez]

Update Sep 24

Next step on this is to create a mockup of the target for focusing design work.

[wendellpiez]

Update Nov 5

Work proceeds here: https://github.com/wendellpiez/metaschema/tree/issue48-m4-schemadocs

[wendellpiez]

Update Dec 3 2020

Work continues; much progress. 85% done on revamp.

Need a little help from @david-waltermire-nist on git alignment.

[david-waltermire]

This issue was addressed by PR #758.