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/commands.py

@@ -17,6 +17,7 @@ from duty_teller.utils.user import build_full_name
 
 
 async def start(update: Update, context: ContextTypes.DEFAULT_TYPE) -> None:
+    """Handle /start: register user in DB and send greeting."""
     if not update.message:
         return
     user = update.effective_user
@@ -47,6 +48,7 @@ async def start(update: Update, context: ContextTypes.DEFAULT_TYPE) -> None:
 
 
 async def set_phone(update: Update, context: ContextTypes.DEFAULT_TYPE) -> None:
+    """Handle /set_phone [number]: set or clear phone (private chat only)."""
     if not update.message or not update.effective_user:
         return
     lang = get_lang(update.effective_user)
@@ -87,7 +89,7 @@ async def set_phone(update: Update, context: ContextTypes.DEFAULT_TYPE) -> None:
 
 
 async def calendar_link(update: Update, context: ContextTypes.DEFAULT_TYPE) -> None:
-    """Send personal calendar subscription URL (private chat only, access check)."""
+    """Handle /calendar_link: send personal ICS URL (private chat only; user must be in allowlist)."""
     if not update.message or not update.effective_user:
         return
     lang = get_lang(update.effective_user)
@@ -136,6 +138,7 @@ async def calendar_link(update: Update, context: ContextTypes.DEFAULT_TYPE) -> N
 
 
 async def help_cmd(update: Update, context: ContextTypes.DEFAULT_TYPE) -> None:
+    """Handle /help: send list of commands (admins see import_duty_schedule)."""
     if not update.message or not update.effective_user:
         return
     lang = get_lang(update.effective_user)