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/api/calendar_ics.py

@@ -102,9 +102,18 @@ def get_calendar_events(
     from_date: str,
     to_date: str,
 ) -> list[dict]:
-    """
-    Return list of {date: "YYYY-MM-DD", summary: "..."} for events in [from_date, to_date].
-    Uses in-memory cache with TTL 7 days. On fetch/parse error returns [].
+    """Fetch ICS from URL and return events in the given date range.
+
+    Uses in-memory cache with TTL 7 days. Recurring events are skipped.
+    On fetch or parse error returns an empty list.
+
+    Args:
+        url: URL of the ICS calendar.
+        from_date: Start date YYYY-MM-DD.
+        to_date: End date YYYY-MM-DD.
+
+    Returns:
+        List of dicts with keys "date" (YYYY-MM-DD) and "summary". Empty on error.
     """
     if not url or from_date > to_date:
         return []