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.

mkdocs.yml

@@ -0,0 +1,29 @@
+# MkDocs configuration for Duty Teller documentation.
+# Build: mkdocs build. Preview: mkdocs serve.
+site_name: Duty Teller
+site_description: Telegram bot for team duty shift calendar and group reminder
+site_url: https://github.com/your-org/duty-teller
+
+docs_dir: docs
+theme:
+  name: material
+  language: en
+
+plugins:
+  - search
+  - mkdocstrings:
+      handlers:
+        python:
+          options:
+            docstring_style: google
+            show_source: true
+            show_root_heading: true
+            heading_level: 2
+
+nav:
+  - Home: index.md
+  - Configuration: configuration.md
+  - Architecture: architecture.md
+  - Import format: import-format.md
+  - Runbook: runbook.md
+  - API Reference: api-reference.md