Add warning message to Gradients and training documentation about ComplexWarnings

[justinpickering]

Context:

• This PR addresses a recurring ComplexWarning issue that appears when using differentiable workflows involving both complex and float types, particularly during backpropagation. Users have reported this issue (see an example here), and internal discussions have concluded that while the warning is generally harmless, it can still be confusing for users.

Description of the Change:

• A warning note has been added to the Gradients and training page in the documentation. The note explains the origin of the ComplexWarning, assures users that it is non-critical in this context, and provides a code snippet to suppress the warning.

Benefits:

• Clarifies to users that the ComplexWarning does not indicate a calculation error, which can help avoid unnecessary troubleshooting.
• Offers an easy method to suppress the warning

Possible Drawbacks:

• Suppressing warnings could potentially hide other, less common issues involving complex numbers in more advanced workflows
• Users may overlook the warning's informational value if they suppress it without fully understanding the implications

Related GitHub Issues:

[mudit2812]

Minor suggestions. Don't forget to add your name to the changelog :)

[CatalinaAlbornoz]

Just left a small comment/question about line 46

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.34%. Comparing base (403c7a3) to head (8dc3d43).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6543   +/-   ##
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  Coverage   99.34%   99.34%           
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  Files         455      455           
  Lines       43193    43232   +39     
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+ Hits        42909    42948   +39     
  Misses        284      284           

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