docs: add normal-normal conjugate example

[kholdrex]

Summary

Adds a Normal-Normal conjugate update to the executable conjugate model examples. The new example shows a known-observation-variance posterior mean update alongside the existing Beta-Binomial and Gamma-Poisson examples.

Changes

• Adds a Normal-Normal posterior helper with validation for empty, non-positive, and non-finite inputs.
• Prints the Normal-Normal posterior mean, standard deviation, and density check in conjugate_models.
• Adds regression coverage for the closed-form posterior and invalid input handling.

Test Plan

• Rust formatting check passed.
• Full Rust test suite passed with all features.
• Clippy passed with warnings denied across all targets and features.
• Example compilation, rustdoc with warnings denied, and benchmark compilation passed.