Metapath Tutorial

[aj-stein-nist]

User Story:

As a Metaschema-enabled tool developer, to know what is required for a Level 1 Capability Metaschema library, I want a tutorial about the use of Metapath in constraints and other parts of Metaschema syntax (like with an index target) I need to know to test constraints and validate cross-document and intra-document references.

Goals:

• Document what Metapath is.
• Document where Metapath is used in Metaschema (constraint, index, et cetera)
• Document how it is used in the relevant locations
• TBD: document the subset of XPath used, specifically
  • the context (. "dot operator")
  • paths
  • supported axes
  • supported functions

Dependencies:

• Allowed Values Tutorial #292 is successfully reviewed and merged

Per discussion of #275, it was agreed upon when creating follow-on issues like #292 that this would be a separate tutorial alongside the allowed values and constraints tutorial, not be contained into complete tutorial.

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]

Removing OSCAL work board per discussion with @david-waltermire-nist.