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/_clipboard-js.scss

@@ -0,0 +1,36 @@
+// clipboard.js
+//
+// JS-based `Copy` buttons for code snippets.
+
+.bd-clipboard {
+  position: relative;
+  display: none;
+  float: right;
+
+  + .highlight {
+    margin-top: 0;
+  }
+
+  @include media-breakpoint-up(md) {
+    display: block;
+  }
+}
+
+.btn-clipboard {
+  position: absolute;
+  top: .5rem;
+  right: .5rem;
+  z-index: 10;
+  display: block;
+  padding: .25rem .5rem;
+  @include font-size(.75em);
+  color: $gray-800;
+  background-color: transparent;
+  border: 0;
+  @include border-radius();
+
+  &:hover {
+    color: $white;
+    background-color: $primary;
+  }
+}