docs: getting started guide (closes #23)

[Dinushi94]

What this PR does

Rewrites docs/getting-started.md as a complete 10-minute getting started guide for engineers who have never heard of Burnless.

Structure

1. What is Burnless (Phase 1 + Phase 2 overview)
2. Install — Homebrew, curl, go install
3. Create your first sre.yaml — wizard walkthrough + annotated output
4. Validate — success output, failure example, CI/CD usage
5. Diff — dry run preview explanation
6. Status — config summary walkthrough
7. Track your first toil event — all flags explained
8. Generate your first report — cost basis and priority explained
9. Export for your manager — CSV output
10. What's next — coming soon features + links

Notes

• Only shows commands that actually work today
• No mention of unbuilt commands (apply, agent) except in coming soon section
• Engineer-to-engineer tone, no marketing fluff
• Every command has a code block with real output

Closes #23