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/_footer.scss

@@ -0,0 +1,35 @@
+//
+// Footer
+//
+
+.bd-footer {
+  @include font-size(.875rem);
+  color: #63707c;
+
+  a {
+    font-weight: 600;
+    color: $gray-700;
+
+    &:hover,
+    &:focus {
+      color: $link-color;
+    }
+  }
+
+  p {
+    margin-bottom: 0;
+  }
+}
+
+.bd-footer-links {
+  padding-left: 0;
+  margin-bottom: 1rem;
+
+  li {
+    display: inline-block;
+
+    + li {
+      margin-left: 1rem;
+    }
+  }
+}