Add favicon and update navigation icons across documentation

- Added a favicon link to all tour pages in the Japanese documentation.
- Updated navigation links to include SVG icons for Home and GitHub.
- Changed language button to include an SVG icon for better visual representation.
- Improved theme toggle button to use SVG icons for light and dark modes.
- Refactored the documentation build commands in the justfile for clarity and consistency.

docs-gen/defaults/config.toml

@@ -3,6 +3,15 @@ title = "My Docs"
 base_url = "https://example.com"
 base_path = ""
 
+# [[nav]]
+# label = "Guide"
+# path = "guide/"
+#
+# [[nav]]
+# label = "GitHub"
+# url = "https://github.com/your/repo"
+# icon = "github"
+
 [i18n]
 default_lang = "en"
 langs = ["en", "ja"]