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/db/models.py

@@ -11,6 +11,8 @@ class Base(DeclarativeBase):
 
 
 class User(Base):
+    """Telegram user and display name; may have telegram_user_id=None for import-only users."""
+
     __tablename__ = "users"
 
     id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
@@ -43,6 +45,8 @@ class CalendarSubscriptionToken(Base):
 
 
 class Duty(Base):
+    """Single duty/unavailable/vacation slot (UTC start_at/end_at, event_type)."""
+
     __tablename__ = "duties"
 
     id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)