Documentation: genui components catalog + prompting guide

[abhithesys]

Context

OpenUI exposes a generative UI components library, but we don't currently document it. Users have no easy way to discover what components are available or how to prompt the LLM to render a specific one, which is blocking wider adoption of the genui layer.

Scope

The goal is a dedicated docs section (or page on the docs site) covering the genui components library end-to-end.

Must have

• Component catalog — every exposed component listed with: short description, supported props/variants, and a screenshot or live preview.
• Prompting guide — recommended patterns and keywords for steering the LLM toward a specific component or layout. Call out anything the system is explicitly tuned for.
• Example gallery — handful of end-to-end examples showing prompt → rendered output across common patterns (forms, dashboards, comparisons, lists, etc.).
• Extension guide — how contributors can add a new component to the library so the LLM can pick it.

[abhithesys]

there's an under-development version of the docs live here. If you've been hitting this issue, we'd really appreciate a review: any gaps, confusing bits, or things you'd expect to see are all useful insights.

cc @eliasinho