validate method
Runs tiers 2 and 3 on doc; markdown is its rendition when the caller
already rendered it.
Implementation
SbpValidation validate(SpecDocument doc, {String? markdown}) {
final rendition = markdown ?? render(doc);
return SbpValidation(
markdown: rendition,
schemaFindings: DocSpecsValidator(schema).validateMarkdown(rendition),
instanceFindings: validateDocument(model, doc),
);
}