Fix: Add scrollbar to the sidebar (#17203)

## Chrome(White mode): 
<img width="1648" alt="Screenshot 2023-04-04 at 10 35 44"
src="https://user-images.githubusercontent.com/44496264/229736193-76c103af-a6a9-463e-968c-f3bd7e84543f.png">

## Chrome(Dark mode): 
<img width="1648" alt="Screenshot 2023-04-04 at 10 35 50"
src="https://user-images.githubusercontent.com/44496264/229736266-568a4650-f054-42ad-bdaf-44ee3f26d40c.png">

## Safari:
<img width="1648" alt="Screenshot 2023-04-04 at 10 35 35"
src="https://user-images.githubusercontent.com/44496264/229736413-ca354f8f-ede4-4f99-8ff8-e1efcd6fa468.png">

## Mozilla:
<img width="1648" alt="Screenshot 2023-04-04 at 10 35 16"
src="https://user-images.githubusercontent.com/44496264/229736453-917edc61-0ae3-4cff-9dcb-33a4d200f6bf.png">

Fixes: #17046

scaladoc/resources/dotty_res/styles/theme/layout/container.css

@@ -19,7 +19,18 @@ p {
   --header-height: calc(8 * var(--base-spacing));
 }
 
+/* Scrollbar */
+
 ::-webkit-scrollbar {
   width: 0;
   background: transparent;
 }
+
+#leftColumn ::-webkit-scrollbar{
+  width: 5px;
+}
+
+#leftColumn ::-webkit-scrollbar-thumb {
+  background: var(--code-syntax-highlighting-scrollbar);
+  border-radius: 2px;
+}