Enable toc sidebar scrolling (#2874)

* Enable toc sidebar scrolling

* Refactor style rules

* Move style rules from 'navigation' to 'sidebar'

* Remove custom scrollbar styles

_sass/minimal-mistakes/_sidebar.scss

@@ -89,6 +89,13 @@
       position: sticky;
       top: 2em;
       float: right;
+
+      .toc {
+        .toc__menu {
+          overflow-y: auto;
+          max-height: calc(100vh - 7em);
+        }
+      }
     }
   }