docs: improve documentation for mutable vs immutable Params

[DLWoodruff]

Added sections for Performance vs. Flexibility, When to use Mutable, Comparison Table (Param vs. Var), and clearer information on updating parameters with set_value().

Fixes # .

Summary/Motivation:

According to a web search, mutable vs. immutable issues are in the top ten troubles for new users (most of the others have something to do with solver status one way or the other)

Changes proposed in this PR:

• An attempt (with the help of Gemini) to create better documentation for Params

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.94%. Comparing base (52a29ae) to head (5a6b420).
⚠️ Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3890      +/-   ##
==========================================
- Coverage   89.94%   88.94%   -1.00%     
==========================================
  Files         902      902              
  Lines      106354   106354              
==========================================
- Hits        95655    94593    -1062     
- Misses      10699    11761    +1062     

Flag	Coverage Δ
builders	29.20% <ø> (-0.01%)	⬇️
default	86.23% <ø> (?)
expensive	35.65% <ø> (?)
linux	86.37% <ø> (-3.05%)	⬇️
linux_other	86.37% <ø> (-1.01%)	⬇️
oldsolvers	28.11% <ø> (+<0.01%)	⬆️
osx	82.71% <ø> (ø)
win	85.81% <ø> (-0.01%)	⬇️
win_other	85.81% <ø> (-0.01%)	⬇️

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