docs: submit private DEX order book walkthrough for #236

[korpo1337]

Summary

Submission artifact for bounty #236: Building a Private DEX Order Book.

Companion repo: https://github.com/korpo1337/midnight-private-dex-orderbook

Includes:

• Compact-style private order-book contract walkthrough
• deterministic Node.js simulator for commitments, aggregate depth, matching and cancellation
• tutorial, video narration script, storyboard and generated slide assets

Verification

Ran in the companion repo:

node src/simulator.mjs
python3 scripts/render_demo_assets.py

Both passed locally.

Closes #236

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