Rename xarray-zarr source and add docs

[sandorkertesz]

This PR renames the xarray-zarr source to zarr. Also adds documentation.

The reason behind the renaming:

The name "xarray-zarr" reflects the fact that the Zarr store is read by Xarray internally. The suggestion is to use a more generic name "zarr". In the future if another Zarr reader is added e.g. based on the Zarr API we could handle it with an extra kwarg in from_source(). E.g.

from_source("zarr", ..., reader="xarray")
from_source("zarr", ..., reader="zarr")

Also consider that none of the existing source names contains/refers to the reader.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 90.70%. Comparing base (5a3c781) to head (202a273).

Files with missing lines	Patch %	Lines
tests/sources/test_zarr.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #723   +/-   ##
========================================
  Coverage    90.70%   90.70%           
========================================
  Files          170      170           
  Lines        12936    12936           
  Branches       616      616           
========================================
  Hits         11733    11733           
  Misses        1015     1015           
  Partials       188      188           

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