Create a public list of validation rules

Right now the JSON Schema is our validation - https://github.com/GSA/openacr/tree/main/schema

We should document how this can be expanded, but also ensure that if others extend the schema, how this can be done to still have a YAML file which validates.

Also, as we add softer validation through the editor interface (via HTML or Javascript), these rules should be documented somewhere on the website.