fix: use correct Bootstrap Icons class for Auto color mode (#6028)

The color-mode dropdown was using bi-circle-fill-half-stroke for the
Auto option, which is a FontAwesome class name. Bootstrap Icons doesn't
ship one with that name, so the icon never rendered.

Replaces it with bi-circle-half (the actual Bootstrap Icons class) in
both the docs MDX snippet and the rendered Astro example.

src/html/components/docs/color-mode.mdx

@@ -70,7 +70,7 @@
               data-bs-theme-value="auto"
               aria-pressed="true"
             >
-              <i class="bi bi-circle-fill-half-stroke me-2"></i>
+              <i class="bi bi-circle-half me-2"></i>
               Auto
               <i class="bi bi-check-lg ms-auto d-none"></i>
             </button>

src/html/pages/docs/color-mode.astro

@@ -145,8 +145,7 @@ const page = "color-mode";
                               data-bs-theme-value="auto"
                               aria-pressed="true"
                             >
-                              <i class="bi bi-circle-fill-half-stroke me-2"
-                              ></i>
+                              <i class="bi bi-circle-half me-2"></i>
                               Auto
                               <i class="bi bi-check-lg ms-auto d-none"></i>
                             </button>