DOC: Make explanation image for Reprs

[ianhi]

What is your issue?

I was helping someone learn to use xarray and they expressed some confusion for understanding the xarray repr (both html and print). They raise good points about the discoverability of what things mean.

i.e.

• what is a bolded coord?
• what are the names in parens
• thrown off by 2d coords looking like they are 1d in the repr
• confusion on what a dimension is on a dataset. not necessarily jus thte dimensions of the data

I think these confusions could be solved with a well made image with annotations on top of an image of the repr. Similar to this anatomy of a figure from matplotlib:

or this legend guide:

(from the amazing: https://stackoverflow.com/a/43439132)