Add CeTZ gallery https://github.com/janosh/diagrams

[janosh]

1. 110+ scientific diagrams, 36 of which have been translated from TikZ to CeTZ (more being added with the goal of reaching parity). visit https://diagrams.janosh.dev and select the cetz tag to see existing CeTZ coverage
2. Covers physics, chemistry, machine learning
3. Includes source code for many diagrams in both TikZ and CeTZ, so useful resource for LaTeX users looking to learn Typst
4. Demonstrates Typst's feature parity with LaTeX for scientific visualization

I added it to the Graphics section of the readme.