Note existing registries for when/if we crawl and index

[lisad]

From https://github.com/sourcemeta/awesome-jsonschema:

Apicurio Registry - A runtime server system for storing and managing API designs and schemas including OpenAPI, AsyncAPI, Avro, and JSON Schema with configurable content rules for evolution control.
Sourcemeta One - A self-hosted JSON Schema microservice that transforms Git repositories into searchable, discoverable schema catalogs with a web explorer, editor integration, schema health checks, and a rich HTTP API.
Sourcemeta Schemas - A public free instance of Sourcemeta One re-offering various open source schema collections.
XRegistry: juancruz noted this one
APIs.guru
SchemaStore