Set schema/profile when parsing YAML
YAML defines numerous profiles. In Antora, we rely only on the simplified YAML schema known as the core (aka json) schema. The core schema provides support for primitive types (null, string, int, boolean, float) as well as sequences (arrays) and maps.
We should set the schema option on the YAML parser to reflect what YAML profile we allow. The default (aka full) schema includes things we don't need, like set, pairs, omaps, binary, timestamps, and map merging. If we do decide that additional capabilities should be enabled, we can handle them as separate requests.