Use WebP for CVD simulation demo in docs instead of indexed PNGs

[kimikage]

cf. #413 (comment)
When I added this demo, I needed to use ImageMagick.jl to read and write compressed images. IIRC, there were no JLLs at that time.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.90%. Comparing base (f0a508b) to head (d6e55da).
Report is 33 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #553      +/-   ##
==========================================
+ Coverage   98.82%   98.90%   +0.07%     
==========================================
  Files          10       10              
  Lines        1281     1281              
==========================================
+ Hits         1266     1267       +1     
+ Misses         15       14       -1     

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[kimikage]

cf. https://discourse.julialang.org/t/what-are-the-best-practices-for-using-incompatible-downstream-packages-in-package-development/114275

[ViralBShah]

Good to merge?