docs(material/sidenav): drawer width in documenation example has content squeezed (#28893)

Fixes the documentation example in the Angular Material 'sidenav' component.
The drawer width in the example does not adequately contain the content

Fixes #28885

(cherry picked from commit 865895e)

Author: lsamboretrorabbit

src/components-examples/material/sidenav/sidenav-drawer-overview/sidenav-drawer-overview-example.css

@@ -1,6 +1,8 @@
 .example-container {
-  width: 400px;
+  width: auto;
   height: 200px;
   margin: 10px;
   border: 1px solid #555;
+  /* The background property is added to clearly distinguish the borders between drawer and main content */
+  background: #eee;
 }