Explain/expand list of current API endpoints/feeds

[brainwane]

https://warehouse.readthedocs.io/api-reference/ (built from docs/api-reference/) should:

• explain, with a little bit more detail, why we call the Simple Project API part of the "legacy API"
• point to pypi.org, not pypi.python.org, in code examples
• mention our RSS feed(s)
• mention any other endpoints we currently offer, such as the JSON feed (Create json.rst #1721 will do part of this)

[brainwane]

I decided that we ought to do this by End User MVP; accurately listing the projects endpoint, the releases endpoint, and other endpoints and feeds we currently serve strikes me as MVP for our end users.

[brainwane]

@di I just realized that the new documentation does not actually mention anywhere the upload endpoint https://upload.pypi.org/legacy/ and I think we should add at least a sentence about it and a pointer to the packaging.python.org tutorial.