Use Hugo for our docs Sass and JS. (#29280)

Now there's only one command needed to run the docs: `npm run docs-serve`.

Also, simplify the npm scripts.

site/assets/scss/_callouts.scss

@@ -0,0 +1,39 @@
+//
+// Callouts
+//
+
+.bd-callout {
+  padding: 1.25rem;
+  margin-top: 1.25rem;
+  margin-bottom: 1.25rem;
+  border: 1px solid $gray-200;
+  border-left-width: .25rem;
+  @include border-radius();
+
+  h4 {
+    margin-bottom: .25rem;
+  }
+
+  p:last-child {
+    margin-bottom: 0;
+  }
+
+  code {
+    @include border-radius();
+  }
+
+  + .bd-callout {
+    margin-top: -.25rem;
+  }
+}
+
+// Variations
+@mixin bs-callout-variant($color) {
+  border-left-color: $color;
+
+  h4 { color: $color; }
+}
+
+.bd-callout-info { @include bs-callout-variant($bd-info); }
+.bd-callout-warning { @include bs-callout-variant($bd-warning); }
+.bd-callout-danger { @include bs-callout-variant($bd-danger); }