docs: landing page ruler

Summary: Adding a small ruler between each section on the landing page.

Reviewed By: jknoxville

Differential Revision: D15148067

fbshipit-source-id: 604f443c06eef16570dcdfbb21e121962b325127

website/static/css/custom.css

@@ -288,6 +288,10 @@ table tr:hover {
   background: #f5f5f5;
 }
 
+footer .sitemap {
+  overflow: hidden;
+}
+
 footer .sitemap .nav-home,
 footer .fbOpenSource {
   opacity: 1;
@@ -369,3 +373,18 @@ footer iframe {
   /* Hide sidebar link to "Features"-pages, because it is linked in header */
   display: none !important;
 }
+
+.content.row {
+  position: relative;
+}
+
+.content.row:not(:last-child)::after {
+  display: block;
+  height: 1px;
+  left: 25%;
+  right: 25%;
+  bottom: -10px;
+  background: #f0f0f0;
+  content: '';
+  position: absolute;
+}