Fix a few docs examples in qnn, fermi and math

[dwierichs]

Context:

Description of the Change:
As title says.
Also reverts a few invalid changes from #5596 in qml.kernels docs.

Benefits:

Possible Drawbacks:

Related GitHub Issues:

[github-actions]

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Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.67%. Comparing base (2f63ca4) to head (972e093).
Report is 244 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5763      +/-   ##
==========================================
- Coverage   99.67%   99.67%   -0.01%     
==========================================
  Files         421      421              
  Lines       40243    39947     -296     
==========================================
- Hits        40114    39817     -297     
- Misses        129      130       +1     

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[trbromley]

Thanks @dwierichs