docs: use new `nuxi module add` command in installation

[danielroe]

This updates the documentation to use the nuxi module add command which should simplify docs a bit and also improve user experience as there's no need to add to nuxt.config manually.

It's documented here.

I may have missed a few spots in the documentation as I'm doing this across the modules ecosystem assisted by the power of regular expressions ✨, so I'd appreciate a review 🙏