docs: add OpenUI integration guide

[AbhinRustagi]

Closes #117

Summary

• Adds a new docs page (app/docs/openui/page.tsx) walking through registering beUI components as an OpenUI library, so a generative UI runtime can render model output with real, animated beUI components (install, defineComponent/createLibrary usage, and a Renderer example).
• Wires the new page into app/sitemap.ts, site search, and the sidebar nav.

Test plan

• bun run typecheck
• bun run lint
• Visit /docs/openui locally and confirm the page renders, code blocks are correct, and sidebar/search links resolve

[starc007]

Thanks for adding this guide — the direction looks useful. A few changes before merging:

• Show beuiLibrary.prompt() being generated and passed to the model; otherwise the documented flow only renders OpenUI Lang but doesn’t explain how to produce it.
• Wire BeButton with useTriggerAction and show Renderer’s onAction; currently generated buttons are inert.
• Replace z.array(z.any()) with a union of the registered component .ref schemas so Stack children are properly described and validated.
• Revert the unrelated bun.lock change since no dependencies changed.