feat: add docker-compose for frontend + quickstart docs

[tvinagre]

Summary

• Add docker-compose.frontend.yml to run Fynd router + swap frontend with a single command
• Add quickstart-frontend docs page for zero-friction onboarding (Docker only, no Rust/CLI needed)
• Users set one env var (TYCHO_API_KEY), run docker compose up, and open localhost:3005

Depends on: propeller-heads/fynd-frontend PR (publishes ghcr.io/propeller-heads/fynd-frontend image)

Test Plan

• docker compose -f docker-compose.frontend.yml config validates successfully
• Both services start with docker compose -f docker-compose.frontend.yml up
• Frontend reaches router via internal Docker DNS (http://fynd-serve:3000)
• Quickstart docs render correctly in GitBook
• SUMMARY.md links resolve

🤖 Generated with Claude Code

[github-actions]

No API Breaking Changes Detected

The PR title signals breaking changes, but cargo-semver-checks found none.
If the breaking change is behavioral, CLI, or config-level (not public Rust API), this is expected.
Otherwise, consider using fix: instead of feat: in the PR title.

[zizou0x]

This should become the main quickstart IMO