[docs] update deterministic to stochastic example

[odow]

I'm giving a few tutorials in October at https://cermics-lab.enpc.fr/seso2023/ and https://julia-users-paris.github.io/workshop/en/.

My plan is to work through a few of these examples, with little exercises at the bottom for people to try.

Preview: https://odow.github.io/SDDP.jl/previews/PR659/tutorial/example_reservoir/

x-ref #658

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.04% ⚠️

Comparison is base (fd2b66f) 92.70% compared to head (fa482e1) 92.67%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #659      +/-   ##
==========================================
- Coverage   92.70%   92.67%   -0.04%     
==========================================
  Files          27       27              
  Lines        3331     3331              
==========================================
- Hits         3088     3087       -1     
- Misses        243      244       +1     

see 1 file with indirect coverage changes

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