Add documentation for zarr.core.group (zarr-developers#2452)

* docstrings

* docstrings for Group

* format

* fix group documentation indentation

* fix group documentation indentation

* add docstrings

* Apply suggestions from code review

Co-authored-by: Joe Hamman <[email protected]>

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Co-authored-by: Joe Hamman <[email protected]>

Author: brokkoli71