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 <[email protected]>
Co-authored-by: Angus Hollands <[email protected]>
Co-authored-by: Tony Fast <[email protected]>
Co-authored-by: Frédéric Collonval <[email protected]>