Updates to Dask page in Xarray docs (#9495)

* First pass at updates to Dask page in Xarray docs

* cleanup

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Fix internal references

* Apply suggestions from code review

Co-authored-by: Tom Nicholas <tom@cworthy.org>
Co-authored-by: Stephan Hoyer <shoyer@google.com>

* Update Dask Array image

* Updates after review

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Some edits

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Tom Nicholas <tom@cworthy.org>
Co-authored-by: Stephan Hoyer <shoyer@google.com>
Co-authored-by: Deepak Cherian <deepak@cherian.net>