docs: add React Native framework guide - #348

Summary

Adds the missing React Native framework guide. RN is featured on the homepage and getting-started tabs and ships a full adapter package, but had only an API reference — no narrative guide — and it was absent from the Framework Guides sidebar.

What's included

• New site/docs/guide/react-native.md — covers the RN adapter's press-driven model (distinct from the web adapters):
  • Explicit ctx (no global context, unlike the web bindings)
  • <Askable> wrapping a single pressable, with nested-ancestor hierarchy
  • useAskableScreen (React Navigation useIsFocused integration)
  • List viewability via useAskableVisibility → onViewableItemsChanged
  • useAskableScrollView measurement + createOnItemLayout
  • Custom sources, streaming/chat (with the RN fetch no-stream-body caveat), agent requests
• Wired into the Framework Guides sidebar and the homepage start-list.

Notes

• Every API was verified against package source. While writing I caught and removed a phantom createAskableStaticSource reference, replacing it with the real AskableContextSource object pattern.
• Docs-only; no code or dependency changes.

Test plan

• vitepress build clean (no dead links) — the same check the CI preview job runs
• CI preview/build green

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