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 🙏

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 42.01%. Comparing base (69ec9f4) to head (a325c1a). Report is 1 commits behind head on main.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #71 +/- ## ========================================== + Coverage 41.82% 42.01% +0.19% ========================================== Files 15 15 Lines 428 426 -2 Branches 32 32 ========================================== Hits 179 179 + Misses 249 247 -2 ```

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