docs: make browser installation substrate-aware

[abrichr]

Summary

• make the browser tutorial explicitly install openadapt[browser]
• document the separate Playwright-driver and Chromium-runtime layers
• show lightweight native Windows and RDP examples that do not install either
• update the installer-script browser invocation

Verification

• 8 documentation validator tests passed
• strict MkDocs build passed

Rollout dependency

Merge/deploy after the launcher browser extra is published. The companion Flow release supplies the base-size reduction.