Document compliance-related JSON Schema extension vocabulary (or vocabularies)

[handrews]

Ideas so far:

• OAS compliance-specific vocabulary that will guide the initial creation of graph nodes and edges
• Custom error reporting vocabulary to improve validation UX (messages, priorities, severities, etc.)
• Generic annotation-driven validation vocabularies (e.g. better URI validation beyond format: uri etc.)
• Possibly an ABNF validation vocabulary, but it is probably simpler as an annotation-driven validation step connected to specific data types (URIs, email, etc.)

Simple value annotation-only vocabularies do not require custom code as of jschon 0.10.