(#21881) Validate ResourceType against a json schema in tests
(#21881) Add documentation for Puppet::Indirection::ResourceType::Parser

This clarifies the use of this indirection somewhat and conforms better to
YARD syntax.
(doc) Add other markdown files to YARD
(#21881) Add API docs to YARD

This adds an api_docs directory with static markdown files which get included
in YARD output. The first thing documented is the resource_type http endpoint.