fix(plots): fix 3D plots in MP7/PRT examples

[wpbonelli]

Reduce plot point color issues (rendering artifacts?) in P03 and other improvements

• use vtk with setup-headless-display-action instead of vtk-osmesa
• export 3D plots as PNG instead of PDF/SVG
• render lines as tubes
• improve zoom/legend font size
• wrap long comments in P04
• try pinning nbconvert==7.16.3 to fix notebook thumbnail rendering
  • Default thumbnail asset not found spatialaudio/nbsphinx#796 (comment)

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