chore(docs): Update PanelResizer hover/focus styles

This makes it a bit more prominent when hovering since the resize cursor
doesn't do much and adds focus styles.

Author: mlaursen

components/PanelResizer.module.scss

@@ -12,7 +12,34 @@
   width: $width;
   z-index: 2;
 
+  &::before {
+    background-color: $rmd-states-focus-shadow-color;
+    bottom: 0;
+    content: '';
+    left: inherit;
+    opacity: 0;
+    position: absolute;
+    top: 0;
+    width: $width;
+  }
+
   &:hover {
     cursor: col-resize;
+
+    &::before {
+      opacity: 1;
+    }
+  }
+
+  @include rmd-utils-keyboard-only(true) {
+    &:focus::before {
+      opacity: 1;
+    }
+  }
+
+  @include rmd-utils-touch-only(true) {
+    &:hover::before {
+      opacity: 0;
+    }
   }
 }