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.

site/sitemap.xml

@@ -0,0 +1,27 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+    <url>
+         <loc>https://github.com/your-org/duty-teller/</loc>
+         <lastmod>2026-02-20</lastmod>
+    </url>
+    <url>
+         <loc>https://github.com/your-org/duty-teller/api-reference/</loc>
+         <lastmod>2026-02-20</lastmod>
+    </url>
+    <url>
+         <loc>https://github.com/your-org/duty-teller/architecture/</loc>
+         <lastmod>2026-02-20</lastmod>
+    </url>
+    <url>
+         <loc>https://github.com/your-org/duty-teller/configuration/</loc>
+         <lastmod>2026-02-20</lastmod>
+    </url>
+    <url>
+         <loc>https://github.com/your-org/duty-teller/import-format/</loc>
+         <lastmod>2026-02-20</lastmod>
+    </url>
+    <url>
+         <loc>https://github.com/your-org/duty-teller/runbook/</loc>
+         <lastmod>2026-02-20</lastmod>
+    </url>
+</urlset>