doc(v3): add v3 roadmap and design document (zarr-developers#1583)

* doc(v3): add v3 roadmap and design document

* Update v3-roadmap-and-design.md

* updates after latest round of reviews

* Update v3-roadmap-and-design.md

Co-authored-by: Norman Rzepka <code@normanrz.com>

* Update v3-roadmap-and-design.md

Co-authored-by: Sanket Verma <svsanketverma5@gmail.com>

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Co-authored-by: Norman Rzepka <code@normanrz.com>
Co-authored-by: Sanket Verma <svsanketverma5@gmail.com>