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 ✅

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

@@            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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