Feature: Add functionality for loading monpol statements dataset

[robin-mader-bis]

This Pull Request adds a load_monpol_statements function for downloading the Monpol statements dataset which has been added to the repository at https://github.com/bis-med-it/gingado/tree/main/assets

Summary of changes:

• Add load_monpol_statements function and related documentation
• Extend internals module and refactor load_CB_speeches to increase reusability