Add _pkgdown.yml with organized reference sections

[keaven]

This PR adds a _pkgdown.yml configuration file that organizes the package's 100+ exported functions into logical, navigable sections on the pkgdown reference page.

Sections

Section	Description
Model Fitting	CSC, FGR, riskRegression, wglm, GLMnet, etc.
Prediction	predictCox, predictRisk, predict.CauseSpecificCox, etc.
Prediction Performance (Score)	Score, IPA, getSplitMethod
Average Treatment Effects	ate and related methods
Weighted GLM Methods	wglm methods
Inference & Influence Functions	iidCox, influenceTest, SE/CI functions
Visualization (ggplot2)	autoplot methods
Visualization (Base Graphics)	plotROC, plotAUC, plotBrier, etc.
Censoring Weights	ipcw, subjectWeights
Data Synthesis & Simulation	synthesize, sim*, sampleData
Datasets	Melanoma, Paquid
Print & Summary Methods	print/summary/coef methods
Conversion to data.table	as.data.table methods
Cox Model Utilities	Low-level cox* helpers
Internal Utilities	C++ functions and misc helpers

Additional changes

• Uses Bootstrap 5 (recommended by pkgdown; BS3 is deprecated)
• Builds cleanly with pkgdown::build_site()

Notes

• The url field is set to ~ (placeholder) since no pkgdown deployment URL exists yet
• Minor warning: hal9001 examples are skipped if the package isn't installed (Suggests dependency)