docs: warn about <NuxtPicture> not supporting different image sources

[Lehoczky]

Related issue: #309

This is a very crucial missing feature, that can bite people later on if they expect the NuxtPicture component to be a drop-in replacement of the native picture element. The warning will help others to find solutions for using multiple image source in the linked issue thread.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.68%. Comparing base (8701991) to head (4fae021).
Report is 50 commits behind head on main.

❗ Current head 4fae021 differs from pull request most recent head f38bee7. Consider uploading reports for the commit f38bee7 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1326      +/-   ##
==========================================
- Coverage   68.20%   59.68%   -8.52%     
==========================================
  Files          76       78       +2     
  Lines        4299     5073     +774     
  Branches      397      407      +10     
==========================================
+ Hits         2932     3028      +96     
- Misses       1339     2015     +676     
- Partials       28       30       +2     

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[danielroe]

Thank you! ❤️

[danielroe]

Thank you! ❤️