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 🙏

Checklist:

• [ ] issue number linked above after pound (#)
  • replace "Closes " with "Contributes to" or other if this PR does not close the issue
• [ ] manually checked my feature / checking not applicable
• [ ] wrote tests / testing not applicable
• [ ] attached screenshots / screenshot not applicable