chore: add changelog and documentation updates

- Created a new `CHANGELOG.md` file to document all notable changes to the project, adhering to the Keep a Changelog format.
- Updated `CONTRIBUTING.md` to include instructions for building and previewing documentation using MkDocs.
- Added `mkdocs.yml` configuration for documentation generation, including navigation structure and theme settings.
- Enhanced various documentation files, including API reference, architecture overview, configuration reference, and runbook, to provide comprehensive guidance for users and developers.
- Included new sections in the README for changelog and documentation links, improving accessibility to project information.

duty_teller/handlers/__init__.py

@@ -6,6 +6,11 @@ from . import commands, errors, group_duty_pin, import_duty_schedule
 
 
 def register_handlers(app: Application) -> None:
+    """Register all Telegram handlers (commands, import, group pin, error handler) on the application.
+
+    Args:
+        app: python-telegram-bot Application instance.
+    """
     app.add_handler(commands.start_handler)
     app.add_handler(commands.help_handler)
     app.add_handler(commands.set_phone_handler)