docs: rewrite the introduction around the project purpose

[mehdibha]

What changed

Rewrites the docs introduction (www/content/docs/(root)/index.mdx) with a simpler, less formal tone focused on the project's purpose rather than technical details. Tone modeled on the shadcn/ui introduction.

• Opens with the purpose: "dotUI is not a component library. It's how you build your own design system" — the familiar problem (every app looks the same, fighting the library) before the solution.
• Adds a 3-step "How it works" (compose → preview → export) replacing the dense feature paragraph.
• Cuts the technical sections: "API philosophy", the primitives comparison, and the foundational-layer (Radix/Base UI/React Aria) discussion, condensed into one short "What's inside" paragraph.
• Keeps the interactive playground demo and the example grid under "Docs that show, not tell".
• "Built for AI" trimmed to two casual sentences as "Works great with AI".

Reviewer notes

Content-only change; the <CompositionAnimation /> component is no longer referenced from this page but its source is untouched.

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Project	Deployment	Actions	Updated
dotui	Ready	Preview	Sep 1, 2026 5:49pm UTC

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.