docs: add KEP-0607 for built-in example notebooks catalog

[ederign]

Summary

• Add KEP proposal for issue feat: Built-in Kale example notebooks in the Launcher #607: a built-in sample notebook catalog accessible from the JupyterLab Launcher
• Covers architecture (filesystem-driven discovery via Jupyter data dirs), catalog YAML schema, materialization flow, frontend components (card grid, tag filtering, conflict dialog), and RPC API surface
• Documents design decisions and alternatives considered (remote catalog, git clone, separate extension, notebook metadata)

Related

• Issue: feat: Built-in Kale example notebooks in the Launcher #607
• Implementation branch: notebook-catalog

Test plan

• KEP renders correctly on GitHub
• TOC links resolve
• Architecture diagram displays properly in monospace

[google-oss-prow]

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