Add Coordinates and Index definitions on Terminology page

[scottyhq]

Reviewing this page today https://xproj.readthedocs.io/en/latest/terminology.html but found myself wanting to have a really basic description of Xarray.Coordinates vs Xarray.Indexes in the docs (in addition to the link to https://docs.xarray.dev/en/latest/generated/xarray.Index.html#xarray.Index which is more technical)

Probably would be best to update the xarray docs and just link to those. I know @keewis started on the Coordinates piece on the data-structures page here https://github.com/pydata/xarray/pull/10323/files. The other page to update is this one https://docs.xarray.dev/en/latest/internals/internal-design.html#internal-design

I'll plan to tackle this, but probably won't get to it for a bit, so just wanted to jot down the links here

[benbovy]

We could also add links to the Index and Coordinate terms already defined in https://docs.xarray.dev/en/latest/user-guide/terminology.html.