feat(📖): add documentation site

[fax1ty]

Summary

• Add apps/docs - Fumadocs + Next.js 16 static site with guides (getting started, rendering, compute, native media, integrations), live WebGPU playgrounds, and API reference
• Add GitHub Pages deploy workflow (.github/workflows/deploy-docs.yml) triggered on docs / webgpu source changes
• Wire yarn build:docs into Turborepo with caching
• Trim README to essentials and point to wcandillon.github.io/react-native-webgpu
• Update package homepage to the docs URL

Test plan

• yarn install --immutable
• yarn build:docs - static export to apps/docs/out
• yarn lint and yarn tsc
• cd apps/docs && yarn dev - browse /, /docs, /api
• Verify playgrounds render (hello triangle, rotating cube, etc.)
• After merge: confirm GitHub Pages deploy workflow runs and site is live at /react-native-webgpu

[fax1ty]

My face when I discovered what this is actually a thing: 🗿

[wcandillon]

I just had a look and this is a very solid start. This is a strong foundation, big thank you for this.
There is one work item I need to finish before shipping this but I think last week the latest we can finish this.

Excited to have some docs finally.