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/personal_calendar_ics.py

@@ -25,10 +25,14 @@ def _parse_utc_iso(iso_str: str) -> datetime:
 
 
 def build_personal_ics(duties_with_name: list[tuple[Duty, str]]) -> bytes:
-    """
-    Build a single VCALENDAR with one VEVENT per duty.
-    duties_with_name: list of (Duty, full_name); full_name is unused for SUMMARY
-    if we use event_type only; can be used later for DESCRIPTION.
+    """Build a VCALENDAR (ICS) with one VEVENT per duty.
+
+    Args:
+        duties_with_name: List of (Duty, full_name). full_name is available for
+            DESCRIPTION; SUMMARY is taken from event_type (duty/unavailable/vacation).
+
+    Returns:
+        ICS file content as bytes (UTF-8).
     """
     cal = Calendar()
     cal.add("prodid", "-//Duty Teller//Personal Calendar//EN")