Formalize documentation of json-key json-key-value

[nikitawootten-nist]

User Story:

As a Metaschema implementor, I need formal documentation as to the expected parsing behavior for Metaschema features, so that I can create interoperable systems that are compatible with Metaschema.

Goals:

Following a thread on Gitter, several underspecified capabilities were surfaced for formal documentation:

• Parsing behavior for JSON document instance Field with a json-key-value @flag-name:
  • Do multiple extraneous properties yield an error? (leading to potential ambiguity or malicious misuse)
  • Is a simple field (no normal flags) with a json-key-value or value @flag-name valid?
• Do json-key-value flags and json-value flags cause a given Field to be considered not simple?
  • In the Java implementation this is no, but "simple" seems to be distinct from "promotable"
• How do json-key and json-key-value interact with collapsibility?
• How is an assembly meant to be parsed if more then one child has a json-key flag?

Dependencies:

N/A

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.