docs: docker-compose example with a persistent volume

[dndrade]

What this does
Adds docs/docker-compose.yml with a bind-mounted ./data volume for persistence, plus a "Docker Compose" section in docs/README.md covering setup, secrets (inline or via .env), running, and verifying persistence.

Why
Closes #4. docker run is in the README, but the first thing a self-hoster reaches for is a compose file with a persistent volume and the env vars (SMOLANALYTICS_PASSWORD, write key) in one place.

Checklist

• make fmt vet race is green
• analytics changes have a test that pins the numbers (n/a — docs-only change)
• no new third-party dependencies (n/a — no code, just a compose file + docs)

Tested locally: docker-compose up -d → logged in via the password gate → docker-compose down → up -d → session and data both persisted without re-login.

[vercel]

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[dndrade]

@Arjun0606, just wanted to follow up on this PR! It's mostly docs and a template setup, so should be pretty straightforward to review. Let me know if you'd like any adjustments!