Summary

Adds a dedicated Atoms guide to cover the missing atom documentation requested in #316.

The new page explains:

• what atoms are in TanStack Store;
• how writable atoms, readonly derived atoms, subscriptions, comparison, and async atoms work;
• how atoms compare with Store;
• where framework adapters fit in;
• links to the relevant API reference pages.

Also adds the page to the Getting Started navigation.

Tests

• pnpm exec prettier --check docs/atoms.md docs/config.json
• node --experimental-strip-types scripts/verify-links.ts
• git diff --check

Note: pnpm run test:docs currently runs node scripts/verify-links.ts, which fails on this local Node 22 setup because the script is a .ts file without a loader. I ran the same link checker with Node's --experimental-strip-types flag instead.

Closes #316.

Summary by CodeRabbit

• Documentation
  • Added comprehensive guide to atoms as reactive primitives, including writable and derived atoms, async atoms with status semantics, custom equality mechanisms, and framework-specific integration examples for React, Preact, and Angular.
  • Enhanced documentation navigation structure with new "Quick Start" and "Atoms" sections in Getting Started.