Port skiplink styling from v3 to v4

docs/assets/scss/_skiplink.scss

+#skippy {
+  display: block;
+  padding: 1em;
+  color: #fff;
+  background-color: $bd-purple;
+  outline: 0;
+
+  .skiplink-text {
+    padding: .5em;
+    outline: 1px dotted;
+  }
+}

docs/assets/scss/docs.scss

@@ -46,6 +46,7 @@ $bd-info:           #5bc0de;
 @import "ads";
 @import "content";
 @import "page-header";
+@import "skiplink";
 @import "sidebar";
 @import "footer";
 @import "component-examples";