docs: add conjugate model examples

[kholdrex]

Summary

Adds executable conjugate Bayesian model examples for common closed-form posterior updates. The examples demonstrate Beta-binomial and Gamma-Poisson updates, then use the resulting bayes-rs distributions for posterior summaries and clearly labeled plug-in predictive checks.

Changes

• Add a deterministic conjugate_models example covering Beta-binomial conversion-rate updates and Gamma-Poisson count-rate updates.
• Add regression coverage for the closed-form posterior parameter updates and plug-in predictive probabilities.

Test plan

• Rust formatting check passed.
• Full Rust test suite passed.
• Example compilation passed.
• Clippy passed with warnings denied.
• Documentation build passed with warnings denied.
• Benchmark compilation passed.