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/import_duty_schedule.py

@@ -19,6 +19,7 @@ from duty_teller.utils.handover import parse_handover_time
 async def import_duty_schedule_cmd(
     update: Update, context: ContextTypes.DEFAULT_TYPE
 ) -> None:
+    """Handle /import_duty_schedule: start two-step import (admin only); asks for handover time."""
     if not update.message or not update.effective_user:
         return
     lang = get_lang(update.effective_user)
@@ -32,6 +33,7 @@ async def import_duty_schedule_cmd(
 async def handle_handover_time_text(
     update: Update, context: ContextTypes.DEFAULT_TYPE
 ) -> None:
+    """Handle text message when awaiting handover time (e.g. 09:00 Europe/Moscow)."""
     if not update.message or not update.effective_user or not update.message.text:
         return
     if not context.user_data.get("awaiting_handover_time"):
@@ -54,6 +56,7 @@ async def handle_handover_time_text(
 async def handle_duty_schedule_document(
     update: Update, context: ContextTypes.DEFAULT_TYPE
 ) -> None:
+    """Handle uploaded JSON file: parse duty-schedule and run import."""
     if not update.message or not update.message.document or not update.effective_user:
         return
     if not context.user_data.get("awaiting_duty_schedule_file"):