feat(docs): add Fumadocs documentation site

[azizbecha]

Summary

New `docs/` workspace serving the library's documentation. Built on Next.js 16 + Fumadocs 16 (App Router + MDX + Orama search).

Pages

• Introduction — quick start with a live ``.
• Installation — yarn/npm/pnpm/bun tabs, peer deps, RN-web notes.
• LinkPreview — full prop table, live demo, states, styling.
• useUrlPreview — signature, return shape, cancellation, validation.
• Caching — defaults, `configureCache`, `invalidateUrl`, `clearCache`.
• API reference — every export with signatures.

Live demos

`LinkPreviewDemo` is a thin client component that imports the actual `LinkPreview` from the library. `next.config.mjs` aliases `react-native` → `react-native-web` for both Turbopack (dev) and webpack (prod).

Workspace plumbing

• Yarn 3 doesn't symlink the root workspace package into a sibling workspace's `node_modules`. `docs/scripts/link-workspace-root.mjs` runs in `postinstall` to create the symlink.
• Root `tsconfig.json` and `tsconfig.build.json` now exclude `docs/` so the library's typecheck and bob build don't trip on Next.js-only globals.
• oxlint/oxfmt ignore `docs/.next/`, `docs/.source/`, `docs/out/`, plus `docs/content/` (oxfmt mishandles MDX with nested code fences inside ``).

Test plan

• `yarn lint` — clean
• `yarn format:check` — clean
• `yarn typecheck` — clean
• `yarn test` — 90/90 pass
• `yarn docs:build` — 10 routes generated (home + 6 docs pages + search + 404 + catch-all)
• Connect Vercel project with Root Directory = `docs` and confirm preview deploys
• Click through every page on the preview deploy and verify live demos render