API Document for WCSAxes (v2)

Here is the latest version of the document describing the proposed API for WCS Axes-like functionality in Astropy. This was developed with @leejjoon after a number of iterations (see #1) and is now ready for comments from @keflavich and @ebressert! Once we have resolved any remaining issues, I'll open a PR on the proper astropy-api repository (and will add you as authors).

The latest rendered API document, with plots, can be viewed here: https://github.com/astrofrog/astropy-api/blob/70b9cee2b7f60f9bc14f8e1de751bdc101db111a/wcs/wcs_api.md