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 🙏

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 57.47%. Comparing base (124da3f) to head (4dc4935).
Report is 36 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@## main #531 +/- ##
===========================================
- Coverage 83.43% 57.47% -25.96% 
===========================================
Files 9 11 +2 Lines 495 729 +234 Branches 30 35 +5 ===========================================
+ Hits 413 419 +6 - Misses 82 310 +228 

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