Add JEP for adding $schema to notebook format (#97)

* Add JEP for adding $schema to notebook format

* wip: update from meeting

* Update add-schema-to-notebook-format.md

change the schema to draft2020-12

* Update URI to align with JEP #108

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Co-authored-by: Filip Schouwenaars <filipsch@users.noreply.github.com>
Co-authored-by: Angus Hollands <goosey15@gmail.com>
Co-authored-by: Tony Fast <tony.fast@gmail.com>
Co-authored-by: Frédéric Collonval <fcollonval@users.noreply.github.com>