feat(docs): add project landing page

[infodrhealthnepal]

Summary

• Adds docs/index.html: a static landing page for Agent Reach, ready for GitHub Pages (Settings → Pages → Deploy from branch → main /docs).
• Mirrors the README's content and voice (Chinese, matching the existing docs): hero + one-line install command, the "agent gets stuck" problem list, a channel status board that mirrors agent-reach doctor output (zero-config / free-core-plus-login / login-required per channel), the "capability layer, not a wrapper" design philosophy with the actual channel-routing file tree, and a security-highlights strip.
• No Python code touched — docs-only change.

Notes

• Content pulled directly from the current README (platform list, backend chains, security section) so it stays consistent with what's already documented.
• Both light and dark themes are handled via prefers-color-scheme.
• Happy to adjust language (e.g. add an English variant), section order, or move this to root index.html instead of docs/ depending on how Pages should be configured.

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