docs: add wrapper documentation for non-sklearn models

[allglc]

Summary

• Add a Getting Started guide for wrapping PyTorch, statsmodels, and saved LightGBM Booster regressors.
• Document shared MAPIE workflows, time-series forecasting with exogenous variables, and wrapper requirements.
• Add the guide to the MkDocs navigation and record the change in HISTORY.md.

Fixes #403, #501, #884