Design a proper favicon for the docs site

[prdai]

The current favicon at `docs/public/favicon.svg` is a quick placeholder — a hand-coded SVG combining the Cloudflare cloud, Drizzle's diagonal slashes, and a key|value cache cell.

Replace it with a properly designed mark.

Notes

• Path: `docs/public/favicon.svg` (wired via Starlight `favicon` in `docs/astro.config.mjs`).
• Keep it legible at 16–32px and on both light and dark browser tabs.
• Match the docs' black & white aesthetic.
• Concept to preserve: KV cache + Cloudflare + Drizzle.
• Consider shipping multiple sizes / a PNG fallback for older clients.

[coderabbitai]

⚠️ Possible Duplicate Issue(s)

• Favicon Not Visible in Dark Mode Browser Themes bbyb.dev#8

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