#2428 Restore v4 docs

The v2 and v3 docs are now restored and integrated into the v4 workspace. This PR brings back the v4 docs saved on the preserve-docs branch of the original v4-spike repository.

It:

• copies the preserved v4 pages into /v4
• keeps the native TypeScript and Python View components
• adds v4 to the version selector and makes it the default
• leaves the existing v2 and v3 navigation unchanged
• updates internal links and redirects for the versioned routes
• restores the v4 SDK-reference conformance tests

The copied page content has only two mechanical changes: internal links are prefixed with /v4, and numeric CardGroup attributes use quoted values for compatibility with the current Mint markdown conversion.

---

Summary by cubic

Restores the Stagehand v4 docs and makes v4 the default docs version. Adds @mdx-js/mdx for MDX parsing in tests, re-enables docs tests, and fixes the Quickstart example.

• New Features

  • Added v4 docs: First Steps, Basics, Configuration, SDK reference, and the v4 index.
  • Updated navigation (docs.json) to include v4 and make it default; README now lists v2, v3, and v4 with v4 as default.
  • Added SDK reference conformance tests and re-enabled docs tests in vitest.config.ts.

• Bug Fixes

  • Corrected the v4 Quickstart script (model config and env variable examples).
  • Resolved merge conflicts and aligned README.md, docs.json, and pnpm-lock.yaml.