Enhance ggRandomForests functionality and documentation

[ehrlinger]

• Updated gg_variable to support randomForest objects and improved handling of training data reconstruction.
• Enhanced gg_vimp to manage cases where importance metrics are unavailable, returning NA placeholders.
• Improved gg_error to compute in-bag training error trajectories for randomForest fits.
• Refined quantile_pts to ensure balanced conditioning intervals for coplots.
• Added a new vignette "Exploring Random Forests with ggRandomForests" for user guidance.
• Updated documentation for all functions to reflect new features and usage.
• Added tests for new functionality and edge cases to ensure robustness.

[codecov]

Codecov Report

❌ Patch coverage is 81.15183% with 36 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.68%. Comparing base (f2aabcb) to head (b3697f9).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
R/gg_error.R	84.40%	17 Missing ⚠️
R/gg_variable.R	77.41%	7 Missing ⚠️
R/gg_vimp.R	82.14%	5 Missing ⚠️
R/gg_partial_rfsrc.R	0.00%	4 Missing ⚠️
R/quantile_pts.R	84.21%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #58      +/-   ##
==========================================
+ Coverage   57.08%   61.68%   +4.59%     
==========================================
  Files          24       24              
  Lines        1687     1832     +145     
==========================================
+ Hits          963     1130     +167     
+ Misses        724      702      -22     

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