doc: use optimistix for IFT in multiphysics demo

[jpbrodrick89]

Relevant issue or PR

• doc: Add multiphysics demo #627

Description of changes

• Made a minimal change to step 7 showing that optimistix implements the implicit function already and linking to other pedagogical sources on the matter.
• Dropped 64-bit enable as Tesseracts are 32 bit, it made no sense to do convergence checks in 64 bit and also led to an optimistix error due to inconsistent dtypes of input and output.

Testing done

• Ran notebook end-to-end
• Tried out nonlinear least square but 4 sensors wasn't enough to show a real

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.03%. Comparing base (93f5fcb) to head (906c585).

Additional details and impacted files

@@                   Coverage Diff                   @@
##           dion/demo-multiphysics     #656   +/-   ##
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  Coverage                   78.03%   78.03%           
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  Files                          39       39           
  Lines                        4685     4685           
  Branches                      770      770           
=======================================================
  Hits                         3656     3656           
  Misses                        720      720           
  Partials                      309      309           

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[dionhaefner]

Looks good to me. Let's just make sure the tests are green.