MPC documentation and citation

[Robbybp]

Improves documentation and adds citation for pyomo.contrib.mpc (and pyomo.dae.flatten).

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By contributing to this software project, I have read the contribution guide and agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ddaa492) 88.34% compared to head (76c970f) 88.34%.
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3148   +/-   ##
=======================================
  Coverage   88.34%   88.34%           
=======================================
  Files         833      833           
  Lines       92730    92730           
=======================================
+ Hits        81918    81919    +1     
+ Misses      10812    10811    -1     

Flag	Coverage Δ
linux	86.24% <ø> (ø)
osx	75.72% <ø> (ø)
other	86.43% <ø> (+<0.01%)	⬆️
win	83.66% <ø> (+<0.01%)	⬆️

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