Add notebook stdin UX design docs

[jlewi]

Summary

• add a design doc for notebook stdin and output UX
• document current behavior, Jupyter comparisons, and a proposed submit-oriented stdin model
• add static HTML mocks for Jupyter-style and Runme-specific input UX

Context

• Refs Not clear when bash cell output waiting for input #99
• Related to Refactor Actions.tsx output rendering and terminal-state handling #190
• Follow-up to Fix renderer-coupled notebook mutations #191

Testing

• not run (docs-only change)

[jlewi]

I'm still reviewing the design;