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/services/import_service.py

@@ -36,7 +36,21 @@ def run_import(
     hour_utc: int,
     minute_utc: int,
 ) -> tuple[int, int, int, int]:
-    """Run import: delete range per user, insert duty/unavailable/vacation. Returns (num_users, num_duty, num_unavailable, num_vacation)."""
+    """Run duty-schedule import: delete range per user, insert duty/unavailable/vacation.
+
+    For each entry: get_or_create_user_by_full_name, delete_duties_in_range for
+    the result date range, then insert duties (handover time in UTC), unavailable
+    (all-day), and vacation (consecutive ranges).
+
+    Args:
+        session: DB session.
+        result: Parsed duty schedule (start_date, end_date, entries).
+        hour_utc: Handover hour in UTC (0-23).
+        minute_utc: Handover minute in UTC (0-59).
+
+    Returns:
+        Tuple (num_users, num_duty, num_unavailable, num_vacation).
+    """
     from_date_str = result.start_date.isoformat()
     to_date_str = result.end_date.isoformat()
     num_duty = num_unavailable = num_vacation = 0