docs: include Preact in supported framework lists

[dogledogle]

What does this PR do?

This is a documentation follow-up to #8, which added Preact support to Canvas UI.

• Add Preact to the supported framework lists across the homepage, documentation, FAQ, and page metadata.
• Update framework counts and copied Markdown content to keep the documentation consistent.

Type of change

• Bug fix
• New component
• Component improvement
• Docs / site
• Other

Checklist

• npx tsc --noEmit passes
• npm run lint passes
• npm run build passes
• Tested in Chrome (with the html-in-canvas flag, if relevant)
• Tested the WebGL overlay fallback (flag off / Safari / Firefox), if a component changed