docs: add 3rd order AD example using Reactant

[avik-pal]

fixes #614 #1094

Not yet functional

[avik-pal]

I noticed users ran into an issue where this takes too long to compile, that is because this is trying to do gradient of the 2nd total derivative, which is expected to be slow (at least based on how this PR implements it).

For simple 3rd order, i.e. gradient(gradient(gradient, this is not really an ideal example and Reactant natively works quite well there

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 37.16%. Comparing base (a2167f2) to head (1de1c52).

⚠️ Current head 1de1c52 differs from pull request most recent head 3242cb1

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Flag	BASE (a2167f2)	HEAD (1de1c52)
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Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1097       +/-   ##
===========================================
- Coverage   82.53%   37.16%   -45.38%     
===========================================
  Files         168       61      -107     
  Lines        6957     2882     -4075     
===========================================
- Hits         5742     1071     -4671     
- Misses       1215     1811      +596     

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