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/static/js/main.js

@@ -33,6 +33,10 @@
   var btn = document.querySelector('.theme-toggle');
   if (!btn) return;
 
+  // Feather Icons: sun (light mode) and moon (dark mode)
+  var sunSVG = '<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><circle cx="12" cy="12" r="5"/><line x1="12" y1="1" x2="12" y2="3"/><line x1="12" y1="21" x2="12" y2="23"/><line x1="4.22" y1="4.22" x2="5.64" y2="5.64"/><line x1="18.36" y1="18.36" x2="19.78" y2="19.78"/><line x1="1" y1="12" x2="3" y2="12"/><line x1="21" y1="12" x2="23" y2="12"/><line x1="4.22" y1="19.78" x2="5.64" y2="18.36"/><line x1="18.36" y1="5.64" x2="19.78" y2="4.22"/></svg>';
+  var moonSVG = '<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><path d="M21 12.79A9 9 0 1 1 11.21 3 7 7 0 0 0 21 12.79z"/></svg>';
+
   function getTheme() {
     var stored = localStorage.getItem('preferred-theme');
     if (stored) return stored;
@@ -45,7 +49,7 @@
     } else {
       document.documentElement.removeAttribute('data-theme');
     }
-    btn.textContent = theme === 'light' ? '\u2600\uFE0F' : '\uD83C\uDF19';
+    btn.innerHTML = theme === 'light' ? sunSVG : moonSVG;
   }
 
   applyTheme(getTheme());