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/errors.py

@@ -14,6 +14,12 @@ logger = logging.getLogger(__name__)
 async def error_handler(
     update: Update | None, context: ContextTypes.DEFAULT_TYPE
 ) -> None:
+    """Global error handler: log exception and reply with generic message if possible.
+
+    Args:
+        update: Update that caused the error (may be None).
+        context: Callback context.
+    """
     logger.exception("Exception while handling an update")
     if isinstance(update, Update) and update.effective_message:
         user = getattr(update, "effective_user", None)