feat: add favicon and logo to documentation

- Introduce favicon.ico for site branding
- Add logo.png for improved visual identity
- Update mkdocs.yml to reference new assets

These changes enhance the visual appeal of the documentation site by
incorporating a logo and favicon, providing a more professional
appearance.

mkdocs.yml

@@ -4,6 +4,8 @@ site_url: https://selfhost-hub.com
 theme:
   name: material
   language: zh
+  logo: assets/logo.png
+  favicon: assets/favicon.ico
   palette: 
     # Palette toggle for automatic mode
     - media: "(prefers-color-scheme)"