#2425 Integrate docs with the v4 workspace

This connects the restored docs to the v4 workspace:

• makes the docs package private and gives it the v4 package version
• uses the repository-pinned Mint CLI
• adds just docs for starting the local docs site
• opens the local docs site automatically
• includes docs validation and broken-link checks in just check and CI
• checks documentation images and videos for alt text
• documents the local development and validation commands
• updates the workspace lockfile

The original Stagehand green is unchanged. It does not meet Mint’s light-background contrast threshold, so the docs check uses mint a11y --skip-contrast. All other accessibility checks remain enabled.

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Summary by cubic

Restores Stagehand docs (v2 and v3) and integrates them with the v4 workspace. Re-enables local docs dev and wires strict docs validation into the root check used by CI.

• New Features

  • just docs runs a repo-pinned mint dev server; no global mint/mintlify needed.
  • Root pnpm check runs the docs check (mint validate, broken links with anchors/redirects/snippets, and mint a11y with contrast skipped).
  • Docs README covers local dev, validation, and publishing.

• Dependencies

  • Migrates from mintlify to mint via the workspace catalog; bumps to mint@4.2.742.
  • Sets @browserbasehq/stagehand-docs to 4.0.0 and private; adds Mint packages to minimumReleaseAgeExclude; updates the lockfile.