fix(docs-infra): better distinguish wrapped headings from other entries in TOC (angular#39092)

Previously, when a heading was longer than the Table of Content's  (TOC)
width and it had to be wrapped into multiple lines, it was hard to
distinguish the subsequent lines from other TOC entries (i.e. other
headings).

This commit makes it easier to visually distinguish wrapped heading
lines from other headings by reducing the spacing between wrapped lines
of the same heading (making it more obvious that they belong together).

PR Close angular#39092

Author: gkalpak

aio/src/styles/2-modules/_toc.scss

@@ -105,9 +105,7 @@
 
     li {
       box-sizing: border-box;
-      @include font-size(12);
-      @include line-height(16);
-      padding: 3px 0 3px 12px;
+      padding: 7px 0 7px 12px;
       position: relative;
       transition: all 0.3s ease-in-out;
 
@@ -129,6 +127,7 @@
         color: lighten($darkgray, 10);
         overflow: visible;
         @include font-size(12);
+        @include line-height(16);
         display: table-cell;
       }
 
@@ -168,11 +167,11 @@
       }
 
       &:first-child:before {
-        top: 13px;
+        top: 15px;
       }
 
       &:last-child:before {
-        bottom: calc(100% - 14px);
+        bottom: calc(100% - 15px);
       }
 
       &:not(.active):hover a:before {