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.

CONTRIBUTING.md

@@ -27,6 +27,14 @@
    ```
    Set `BOT_TOKEN` and any other variables as needed (see README).
 
+5. **Documentation (optional)**  
+   To build and preview the docs (MkDocs + mkdocstrings):
+   ```bash
+   pip install -e ".[docs]"
+   mkdocs build
+   mkdocs serve   # preview at http://127.0.0.1:8000
+   ```
+
 ## Running tests and linters
 
 - **Tests** (from repository root; package is `duty_teller`, no `src/`):
@@ -54,3 +62,7 @@ Use [Conventional Commits](https://www.conventionalcommits.org/), e.g.:
 - `docs: update README env section`
 
 Submit changes via pull requests (Gitea Flow); reviews consider functionality, code quality, and security.
+
+## Releases
+
+When cutting a release, update [CHANGELOG.md](CHANGELOG.md) with the new version and list changes under Added / Changed / Fixed / Security as appropriate (see [Keep a Changelog](https://keepachangelog.com/)).