docs: include placeholder prop in nuxt-img props

[treboryx]

No description provided.

[codecov-commenter]

Codecov Report

Merging #533 (b75db69) into main (a05bbe9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #533   +/-   ##
=======================================
  Coverage   58.73%   58.73%           
=======================================
  Files          34       34           
  Lines         647      647           
  Branches      218      218           
=======================================
  Hits          380      380           
  Misses        266      266           
  Partials        1        1           

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[pi0]

Thanks!