Make Table of Contents scrollable (#1696)

Inspired by https://stackoverflow.com/a/57040610/1283519, this change makes the Table of Contents scrollable (the menu as seen on on the left side in a non-mobile browser).

Fixes https://github.com/esphome/issues/issues/2088.

_static/custom.css

@@ -13,6 +13,11 @@ div.body {
   min-width: initial;
 }
 
+div.sphinxsidebar {
+    max-height: 100%;
+    overflow-y: auto;
+}
+
 img.component-image {
   border: none;
   vertical-align: middle;