Documentation revamp

[BatyLeo]

• [x] Use DocStringExtensions.jl
• [x] Separate API between specific problems, losses, optim layers and internals
• [x] Background page for math (optim + ML), focus on discrete functions rather than linear programs
  • [ ] explain chain rules
  • [ ] add math details on each feature
• [ ] 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)
  • [ ] remove grid graphs from the graphs tutorial
  • [ ] add a tutorial with JuMP
  • [ ] showcase all features in the tutorial
  • [ ] demonstrate use of kwargs

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (5584bbe) 89.78% compared to head (20ff86e) 89.76%. Report is 3 commits behind head on main.

Files	Patch %	Lines
src/perturbed/additive.jl	0.00%	2 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #102 +/- ## ========================================== - Coverage 89.78% 89.76% -0.02% ========================================== Files 24 24 Lines 695 694 -1 ========================================== - Hits 624 623 -1 Misses 71 71 ```

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