docs: add event setup guide for workshop facilitators

[devin-ai-integration]

Summary

Adds docs/event-setup-guide.md — a single-page, end-to-end walkthrough for facilitators setting up a workshop event. Consolidates the setup workflow currently spread across the README, facilitator guide, templates, and individual script --help output into a linear step-by-step guide covering:

• Prerequisites — API key, GitHub PAT creation, auth verification
• Phase 1 (pre-event) — mirror repos, create config JSON, provision Devin org, invite participants, verify environment setup, deploy PII checks, provision runtime resources
• Phase 2 (day-of) — setup checklist, opening/closing scripts, common issues table
• Phase 3 (post-event) — cleanup scripts, org teardown, follow-up checklist
• Quick reference — all scripts with key flags in one table
• Troubleshooting — ACU limits, create_as_user_id, git permissions, PAT approval, GHES cross-host auth

All content is sourced from existing repo docs and scripts — no new functionality, just a consolidated guide.

Link to Devin session: https://partner-workshops.devinenterprise.com/sessions/ada1c41829ca4b8c82dadea6b44cfb75
Requested by: @DhrovS

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[devin-ai-integration]

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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