Add simple example "Low-rank matrix completion"

[jd-foster]

From https://discourse.julialang.org/t/nuclear-norm-minimization-objective-using-jump/122869

Co-authored-by: @m-laprise
Co-authored-by: @odow

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.58%. Comparing base (7eeee45) to head (25da02c).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3888   +/-   ##
=======================================
  Coverage   99.58%   99.58%           
=======================================
  Files          43       43           
  Lines        6031     6031           
=======================================
  Hits         6006     6006           
  Misses         25       25           

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