Add a terminology page to docs for "translating" between CF and xarray terminology

[jthielen]

xarray itself has a very helpful Terminology page that gives definitions for the key objects/concepts in xarray. As that page notes, these differ somewhat from CF. Since cf-xarray further acts to bridge the gap between xarray and the CF conventions, it may be helpful to have a sort of "terminology translation" page to the cf-xarray docs to alleviate confusion. For example: xarray uses "dimension coordinate" and "non-dimension coordinate" whereas CF uses "coordinate variable" and "auxiliary coordinate variable".