docs: improve the Working with Chrome Extensions section (#8321)
docs: improve the Working with Chrome Extensions section (#8321)

- Added a note to mention the experimental Chrome headless mode can
  also be used.
- Improved the example code to wait for the background page to be
  ready. The previous example sometimes didn't work, since the
  background page would take a split-second to load.
- Added a note to mention how to find the background ServiceWorker for
  Chrome MV3 extensions.
puppeteer/puppeteer,puppeteer/puppeteer,puppeteer/puppeteer,puppeteer/puppeteer