Updating prototype to use `tagging` and `tagging-setup` keys

The [prototype](https://latex3.github.io/tagging-project/documentation/prototype-usage-instructions.html) page could be updated to use the new `tagging` and `tagging-setup` keys instead of `testphase` and `\tagpdfsetup`. This is basically what's recommended in ltnews41 so it would be good to have the webpage match.