Modified transform parameter in plot_forest to accept dictionary values

[lucifer4073]

Closes #2117

Key Features:

• Callable transform: Applies a single transformation to all variables.

  az.plot_forest(data, transform=lambda x: x ** 2)

• Dictionary transform: Allows variable-specific transformations.

  az.plot_forest(data, transform={"theta": np.exp, "mu": np.log})

Additional Changes:

• Added test cases to validate the feature.
• Documentation updated to follow the NumPy style guide.
• Example script added to demonstrate usage

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📚 Documentation preview 📚: https://arviz--2403.org.readthedocs.build/en/2403/

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 86.72%. Comparing base (adb2fef) to head (c8189dc).

Files with missing lines	Patch %	Lines
arviz/plots/forestplot.py	91.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2403   +/-   ##
=======================================
  Coverage   86.72%   86.72%           
=======================================
  Files         124      124           
  Lines       12908    12919   +11     
=======================================
+ Hits        11194    11204   +10     
- Misses       1714     1715    +1     

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[lucifer4073]

Hi @OriolAbril, it would be great if you could review my PR.