add design document for "named-array" (#8073)

* Add draft design document

* add appendix

* update API surface

* [skip-ci] add references/links

* [skip-ci] update doc

* Update design_notes/xarray_lite_design_doc.md

Co-authored-by: Deepak Cherian <dcherian@users.noreply.github.com>

* [skip-ci] update name

* [skip-ci] Update design_notes/named_array_design_doc.md

Co-authored-by: Anderson Banihirwe <13301940+andersy005@users.noreply.github.com>

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Co-authored-by: Deepak Cherian <dcherian@users.noreply.github.com>