schemacode: Improve API documentation for the schema object

[effigies]

Docs for the Namespace class show the use of .items/keys/values(level=2).

https://bidsschematools.readthedocs.io/en/latest/api/bidsschematools.types.namespace.html#bidsschematools.types.namespace.Namespace

It looks like we don't make it clear that that's what you're getting from load_schema.

Originally posted by @effigies in https://github.com/bids-standard/python-validator/issues/12#issuecomment-2375113215

---