public Optional<String> getSchemaKeyword() {
        return Optional.ofNullable(schemaKeyword);
    }
Gets the "$schema" keyword of the document.

@return Returns the optionally defined $schema.
@see <a href="https://json-schema.org/latest/json-schema-core.html#rfc.section.7">$schema</a>
smithy-jsonschema/src/main/java/software/amazon/smithy/jsonschema/SchemaDocument.java