Docs revamp

[gdalle]

High priority:

• Use https://github.com/JuliaDocs/DocStringExtensions.jl
• Background page for math (optim + ML), focus on discrete functions rather than linear programs
• Guide for choosing the right tool (as in https://scikit-learn.org/stable/tutorial/machine_learning_map/index.html)
• Lots of tutorials (remove GridGraphs from the graph tuto)
• Separate API between specific problems, losses, optim layers and internals
• Specify private API

Low priority:

• Way to test without gradient descent, or to avoid testing the full tensor problem * layers
• Flux or Lux?