Add content to cumulusci.yml Pydantic models and JSON Schema

[jlantz]

Added title, description, and examples to all Pydantic models for the cumulusci.yml json schema to provide better help information in IDEs.

There are a few places where I added enums.

[jstvz]

@jlantz Looks like the some test failures due to a schema mismatch. I'll push a copy of your branch to the remote to resolve the lint failure afterwards.