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.

docs/api-reference.md

@@ -0,0 +1,43 @@
+# API Reference
+
+Generated from the `duty_teller` package. The following subpackages and modules are included.
+
+## Configuration
+
+::: duty_teller.config
+
+## API (FastAPI and auth)
+
+::: duty_teller.api
+::: duty_teller.api.app
+::: duty_teller.api.dependencies
+::: duty_teller.api.telegram_auth
+::: duty_teller.api.calendar_ics
+::: duty_teller.api.personal_calendar_ics
+
+## Database
+
+::: duty_teller.db
+::: duty_teller.db.models
+::: duty_teller.db.schemas
+::: duty_teller.db.session
+::: duty_teller.db.repository
+
+## Services
+
+::: duty_teller.services
+::: duty_teller.services.import_service
+::: duty_teller.services.group_duty_pin_service
+
+## Handlers
+
+::: duty_teller.handlers
+::: duty_teller.handlers.commands
+::: duty_teller.handlers.import_duty_schedule
+::: duty_teller.handlers.group_duty_pin
+::: duty_teller.handlers.errors
+
+## Importers
+
+::: duty_teller.importers
+::: duty_teller.importers.duty_schedule