docs: refresh README for beginner-friendly Boo launch

[Ansub]

What changed

• Updated hero positioning to: Boo — Adding taste to your terminal.
• Rewrote top-level copy to be less technical and more beginner-friendly
• Refocused feature section on user outcomes (themes, setup, splash, doctor, safe install lifecycle)
• Added simple beginner quick-start examples
• Synced README theme references and commands with current Boo capabilities

Why

For today's launch, README should speak to new terminal users first, while still being accurate for advanced users.

Notes

• No code behavior changes
• Docs-only PR

[Ansub]

✅ APPROVED

README launch refresh looks strong and aligned with current Boo behavior.

What I verified

• New positioning/tagline is consistent and beginner-friendly.
• Quick-start snippets map to real commands (boo theme, boo font, boo opacity).
• Theme/docs updates align with current theme set and command surface (including theme create/delete examples).
• No misleading or breaking documentation regressions found in this diff.

Non-blocking note

• PR is currently marked UNSTABLE on GitHub (likely checks/branch state). Content itself is good from a docs correctness standpoint.

Approved from documentation correctness/clarity perspective.