docs(readme): polish for public release

[damianrosellen1]

Summary

Restructures and tightens the root README for the public release without changing the meaning of any existing claim. The repo's technical surface is unchanged.

Highlights

• Stronger lead — one-line tagline + a row of shields.io badges (Next, React, Sanity, TypeScript, Tailwind, Biome, license). No emoji.
• "Two flavours" section right under the lead, surfacing the variant/document-level branch and linking to PR [variant — do not merge] Document-level translation (alternative to field-level i18n) #62 so visitors immediately understand the parallel variant.
• "What you get" reflows the previous dense bullet sprawl into themed paragraphs (Next.js app, Studio, Media, Revalidation, SEO, Hardened defaults, Typed GROQ, Tailwind v4) — reads like feature cards, not a checklist.
• Quickstart split into clean steps instead of one bash block with inline numbered comments.
• "Architecture in brief" replaces the long "Core APIs & modules" tables; depth moves into the per-folder READMEs (already linked from "Going deeper").
• New "License" footer; "Requirements" moves to the bottom (you don't read it before you're sold on the repo).
• Fixed an orphan link to packages/sanity-dataset-resolve/README.md (file doesn't exist; pointed at the inline-documented src/index.ts instead).
• The pnpm strip-readmes utility gets one explicit mention so consumers know the documentation density is opt-in.

Net

• 271 → 213 lines.
• 0 emoji.
• All external + internal links resolve.

Test plan

• pnpm lint clean (only the pre-existing useOptionalChain warning, unrelated)
• All linked files exist (sanity-checked)
• Badge URLs are static shields.io references (no auth-gated endpoints)