Long overdue, but still very much work in progress!

I've just added the outline, where I'd like to show what the index refactor addresses and/or enables from the basic to the most advanced use cases. The goal is also to refer to this blog post when commenting on new Xarray issues related to this topic.

Once it is ready, probably best would be to wait for an Xarray stable release before publishing it (ideally with the documentation further improved).