docs(survival): improve parametric survival chapter narrative and math

- Expand exponential and Weibull derivations with line-by-line annotations
- Add proof and example blocks for Weibull hazard properties theorem
- Clarify mathematical relationship between PH and AFT frameworks
- Add variable descriptions for leukemia dataset and compare R outputs
- Synthesize discussion on left-truncation and interval-censoring

Co-authored-by: dem-extra1 <112029334+dem-extra1@users.noreply.github.com>

Author: google-labs-jules[bot]