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/group_duty_pin.py

@@ -91,6 +91,7 @@ async def _schedule_next_update(
 
 
 async def update_group_pin(context: ContextTypes.DEFAULT_TYPE) -> None:
+    """Job callback: refresh pinned duty message and schedule next update at shift end."""
     chat_id = context.job.data.get("chat_id")
     if chat_id is None:
         return
@@ -117,6 +118,7 @@ async def update_group_pin(context: ContextTypes.DEFAULT_TYPE) -> None:
 async def my_chat_member_handler(
     update: Update, context: ContextTypes.DEFAULT_TYPE
 ) -> None:
+    """Handle bot added to or removed from group: send/pin duty message or delete pin record."""
     if not update.my_chat_member or not update.effective_user:
         return
     old = update.my_chat_member.old_chat_member
@@ -185,6 +187,7 @@ def _get_all_pin_chat_ids_sync() -> list[int]:
 
 
 async def restore_group_pin_jobs(application) -> None:
+    """Restore scheduled pin-update jobs for all chats that have a pinned message (on startup)."""
     loop = asyncio.get_running_loop()
     chat_ids = await loop.run_in_executor(None, _get_all_pin_chat_ids_sync)
     for chat_id in chat_ids:
@@ -194,6 +197,7 @@ async def restore_group_pin_jobs(application) -> None:
 
 
 async def pin_duty_cmd(update: Update, context: ContextTypes.DEFAULT_TYPE) -> None:
+    """Handle /pin_duty: pin the current duty message in the group (reply to bot's message)."""
     if not update.message or not update.effective_chat or not update.effective_user:
         return
     chat = update.effective_chat