Open deepnote file

[Artmann]

• Adds support for opening .deepnote files.
• Adds a command for switching between notebooks.

Summary by CodeRabbit

• New Features

  • Deepnote first-class support: new notebook type, open/save serializer, select/switch notebook command and quick-pick UI, activation wiring, data/output conversion, MIME handling, and widget preload.

• Documentation

  • Added Deepnote guidance file; updated issue/bug templates and contact links to point to Deepnote resources; added localization entry.

• Tests

  • Unit tests for data conversion, selector, notebook manager, and output round-trips; small lint directive tweak.

• Chores

  • CI/workflow reorganization: removed legacy workflows and added streamlined CI and dependency-check workflows.