docs: replace chrome extension TODO placeholder

[hiSandog]

Summary

• replace the visible @TODO placeholder in the Chrome Extension docs page
• add a concise bilingual integration guide that explains the token, window.PAGE_AGENT_EXT, and the main execution callbacks
• keep the existing API reference and source link intact

Validation

• npm run build:website --workspace=@page-agent/website

[CLAassistant]

All committers have signed the CLA.

[gaomeng1900]

Thanks for the contribution.
I’m closing this PR because it was flagged by our review checks as high risk for insufficient human ownership.
This project does not accept bot/AI-generated contributions without substantial human involvement and review.