[docs] add multi-commodity network example

[odow]

I went to write a nested-summation tutorial (#2348 (comment)), and I realized that suggesting the relational approach in the abstract is a bit vague.

Motivated by https://discourse.julialang.org/t/creating-variables-from-a-vector-of-tuples-and-naming-them/103525, I extended the multi-commodity flow problem to a network. It lets us show off a lot more techniques that should be helpful to others.

https://jump.dev/JuMP.jl/previews/PR3491/tutorials/linear/multi_commodity_network/

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01% 🎉

Comparison is base (a157126) 98.11% compared to head (9bb3e05) 98.13%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3491      +/-   ##
==========================================
+ Coverage   98.11%   98.13%   +0.01%     
==========================================
  Files          37       37              
  Lines        5531     5532       +1     
==========================================
+ Hits         5427     5429       +2     
+ Misses        104      103       -1     

Files Changed	Coverage Δ
src/print.jl	99.76% <100.00%> (+0.23%)	⬆️

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