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/index.md

@@ -0,0 +1,13 @@
+# Duty Teller
+
+Telegram bot for team duty shift calendar and group reminder. The bot and web UI support **Russian and English**.
+
+## Documentation
+
+- [Configuration](configuration.md) — Environment variables (types, defaults, examples).
+- [Architecture](architecture.md) — Components, data flow, package relationships.
+- [Import format](import-format.md) — Duty-schedule JSON format and example.
+- [Runbook](runbook.md) — Running the app, logs, common errors, DB and migrations.
+- [API Reference](api-reference.md) — Generated from code (api, db, services, handlers, importers, config).
+
+For quick start, setup, and API overview see the main [README](../README.md).