Add Documentation for Local Refinements for Metaschema Syntax

[aj-stein-nist]

User Story:

As a NIST OSCAL developer, in order to better understand how to locally define flags, allowed-values, and json-keys, I would like more documentation on how to use them in Metaschema definitions.

Goals:

• Replace TBDs with documentation for flags (addressed in PR Feature improve documentation #329)
• Replace TBDs with documentation for allowed-values (addressed in Clarify specification for processing combined allowed-value constraints #411 and by PR Clarify allowed-values constraints #413)
• Replace TBDs with documentation for json-keys (addressed in Formalize documentation of json-key json-key-value #316)

Dependencies:

{Describe any previous issues or related work that must be completed to start or complete this issue.}

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.

{The items above are general acceptance criteria for all User Stories. Please describe anything else that must be completed for this issue to be considered resolved.}

[aj-stein-nist]

Per discussion with team reorganization, I will move this Metaschema issue off the NIST OSCAL workboard.