Improve the `gendocs` management command

[bgrant]

I tried out gendocs recently, and here's a wishlist of features I wish it had:

• gendocs doesn't list if a parameter is required or not (seems easy to add, since the information is there in DDA)
• gendocs doesn't provide docs for members of JSON objects
• gendocs doesn't produce docs for responses
• gendocs outputs types with a Python repr, like <class 'str'> . This makes its output invalid JSON, and it would probably be more useful to output the JSON type anyway (rather than the Python type)
• gendocs doesn't produce docs that span how to use the API, like "you need to follow up this API call with this other one". Maybe we could document the API this way, or maybe gendocs isn't the place for it.
• gendocs seems to add request as a field on every endpoint
• gendocs generates different endpoint docs for "behaviorally routed" classes, but it doesn't specify how they're different - for example '"type" is the field that dispatches between these'
• gendocs doesn't say / know if a field is deprecated
• gendocs doesn't say / know if a field or endpoint is "private" (so we could exclude it from the docs, optionally). Alternatively, we could have different doc "targets" (internal vs. external for example)
• It would be nice if we could specify output formats (like Markdown)
• gendocs doesn't handle parameters inside JWTs