[New Article] Where and how to use API documentation

[magicznyleszek]

This should describe:

• where to find api/v2 (e.g. https://kf.kobotoolbox.org/api/v2/ for https://kf.kobotoolbox.org/)
• how the tree of nested endpoints work, including the <uid> parameters etc.
• how to use the form at the bottom
• magic tricks like adding .json at the end (or ?format=)

Ideally this should include a better description for the root endpoint: /api/v2/.