Additional constraints epic

[aj-stein-nist]

User Story:

As a Metaschema developer, in order to understand how explain, design, and implement usage of Metaschema-based models, I need an updates to the specification on the Metaschema website re enumerated values to document how to use all constraint types, not just the most common allowed-values enumeration constraint type documented in #411 work.

This will document those constraints currently identified in the Metaschema XML Schema that defines the syntax of a module.

Goals:

Document all the constraint types in metaschema, including:

• allowed-values (completed in Clarify specification for processing combined allowed-value constraints #411)
• Document has-cardinality constraint #546
• Document expect constraint #489
• Document index and index-has-key constraint #490
• Document is-unique constraint #491
• Document matches constraint #492
• Document constraint <message/> element #496

(NOTE: I added a check to confirm the work is almost complete or in the process of final review, unchecked items are free for other members of the maintainer team to pick up.)

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.

{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]

I checked off ones that are far enough in review they are "done" and I might want some help starting others. @Arminta-Jenkins-NIST, when you are back next week, do you think we could work together to document is-unique and match for #491 and #492 respectively? I would like some fresh eyes to make sure what I wrote before makes sense: these should be very straightforward given the explanation in the others.

[aj-stein-nist]

Alongside #545, I am adding #546 to this epic as it fell through the cracks. I can handle this quickly later today or early tomorrow morning.