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/_content.scss

@@ -0,0 +1,98 @@
+// stylelint-disable no-duplicate-selectors, selector-max-combinators, selector-max-compound-selectors, selector-max-type, selector-no-qualifying-type
+
+//
+// Automatically style Markdown-based tables like a Bootstrap `.table`.
+//
+
+.bd-content {
+  order: 1;
+
+  // Hack the sticky header
+  > h2[id],
+  > h3[id],
+  > h4[id] {
+    pointer-events: none;
+
+    &::before {
+      display: block;
+      height: 6rem;
+      margin-top: -6rem;
+      content: "";
+    }
+  }
+
+  // Override Bootstrap defaults
+  > .table {
+    max-width: 100%;
+    margin-bottom: 1.5rem;
+
+    @include media-breakpoint-down(md) {
+      display: block;
+      overflow-x: auto;
+
+      &.table-bordered {
+        border: 0;
+      }
+    }
+
+    th,
+    td {
+      &:first-child { padding-left: 0; }
+      &:not(:last-child) { padding-right: 1.5rem; }
+    }
+
+    // Prevent breaking of code (e.g., Grunt tasks list)
+    td:first-child > code {
+      white-space: nowrap;
+    }
+  }
+}
+
+.bd-content-title {
+  display: block;
+  pointer-events: auto;
+}
+
+//
+// Docs sections
+//
+
+.bd-content {
+  > h2 {
+    @include font-size($h2-font-size);
+  }
+
+  > h3 {
+    @include font-size($h3-font-size);
+  }
+
+  > h4 {
+    @include font-size($h4-font-size);
+  }
+
+  > h2:not(:first-child) {
+    margin-top: 3rem;
+  }
+
+  > h3 {
+    margin-top: 1.5rem;
+  }
+
+  > ul li,
+  > ol li {
+    margin-bottom: .25rem;
+  }
+}
+
+.bd-title {
+  margin-bottom: .5rem;
+  @include font-size(3rem);
+}
+
+.bd-lead {
+  @include font-size(1.5rem);
+  font-weight: 300;
+}
+
+.bd-text-purple { color: $bd-purple; }
+.bd-text-purple-bright { color: $bd-purple-bright; }