new figure aligned with the docs

[andreacassioli]

Fixes #185

I made a new figure identical to the old except that\

• source and sink are swapped in color as per the docs
• the image is now a png as most of the rest of the docs

[jeremy-murphy]

Thank you! I see what you mean about the black/white inversion.

How did you make the diagram? I'm curious how you got that hand-drawn style and whether changing it would be easy or difficult. 😅 I personally would prefer to stick with a formal style, and preferably generated by Graphviz so that they are easy to reproduce, although that is a secondary consideration.

[andreacassioli]

I have used this tool https://excalidraw.com/. Indeed it is not standard, but it takes 2 minutes!

I did not find any guideline and I could not see graphviz files for the other images, so I assumed people just made figures as needed.

[jeremy-murphy]

I make a lot of guidelines up as I go. 😆 One day I will put them in the README.

I'll merge this since it does fix a glaring logical discrepancy and we can worry about style some other time.

[jeremy-murphy]

Thanks!