Update AssignmentCalendar and Rubric docs for cleaner OpenAPI doc generation

[battis]

I have an extensive list of places where the Canvas LMS API documentation includes text or values that that don't parse into clean OpenAPI specifications. Not a huge deal, but if the OpenAPI spec were cleaner, it would be easier to automate building a more complete, capable client. So this is my first test pass at tweaking the documentation.

I have tweaked three specific things:

1. List available account calendars says that it returns an array of AccountCalendars. It does not -- it returns an object containing that list and a total. I created a new model AccountCalendarList to describe this result and updated the documentation.
2. Creates a rubric using a CSV file in the JSON spec file incorrectly stated the model name. It is now correct.
3. The Outcomes model references a RubricRating, but does not define it, again creating problems parsing the JSON spec file. RubricRating had already been duplicated once into the Assignments API, so I duplicated it again.

Test Plan:

• Build and validate the OpenAPI docs
• Test that the generated JSON docs that would be hosted at https://example.instructure.com/docs/api/account_calendars.json, https://example.instructure.com/docs/api/outcomes.json, and https://example.instructure.com/docs/api/rubrics.json are valid
• Test that the developer documentation linked to above generates cleanly