Forecast milestone final review / clean-up

[catkins-miso]

• Review forecast proposal / snapshot endpoint documentation for clarity and correctness #82
• deprecate "missing" forecast endpoints, implement proposal status response #74
• cover all the media types related to forecasting in the docs
• Add response examples for partially valid forecast proposals, partial PATCH submissions, and all client errors
• Check required fields in schemas (replaces Audit "required" fields across the spec #4 in the forecast milestone)
• Prepare blog post / article / overview to announce
• cut a release!

[catkins-miso]

@getorymckeag can you think of anything else that needs to be addressed before? I think we want to define the scope of this "1.0" to be something like "the forecast surface of the API is considered stable, but now we need feedback. Planned work is to write the conformance suite tests, and the likelihood of minor changes based on feedback is high."