docs: make README beginner-first with quickstart and troubleshooting

[Ansub]

Why

The README had strong content, but new users had to scan multiple sections before getting a reliable first success. This PR optimizes for a beginner-first setup flow.

What Changed

• Added a top-level Start Here (2 to 5 minutes) section with copy/paste commands.
• Added explicit expected results after install/verify/customization steps.
• Reorganized setup into Prerequisites, Install, and Verify It Worked.
• Reworked troubleshooting by symptom (command not found, changes not applying, doctor issues).
• Consolidated command docs under Common Tasks with clearer labels.
• Moved reload internals to Advanced Notes to reduce initial cognitive load.
• Surfaced backup/restore safety notes near install.

Scope

• Docs-only change (README.md)
• No CLI or runtime behavior changes

Reviewer Checklist

• Follow the Start Here block in order and confirm it reads clearly for first-time users.
• Confirm command examples are valid (boo theme, boo opacity, boo prompt, boo doctor, boo reload).
• Confirm troubleshooting covers the top failure paths with direct next steps.
• Confirm advanced details are discoverable but not blocking the first run.

Testing

• Manual docs QA only (no code changes)

Follow-Up Ideas (separate PR)

• Add screenshots/GIFs for boo status and a theme switch.
• Add a compact command glossary table for absolute beginners.
• Add a one-screen "ultra-quickstart" at the top for skimmers.