Fixes links accessibility by adding underlines (#2497)

Summary:
Closes #2494

Pull Request resolved: #2497

Differential Revision: D22579546

Pulled By: claudiopro

fbshipit-source-id: 32d78fd6296013878820084ba8fd1e6343285ccb

Author: R Nabors

website/src/css/custom.css

@@ -30,4 +30,8 @@
 
 .container {
   filter: grayscale(100%);
-}
+}
+
+.main-wrapper a:link:not(.button) {
+  text-decoration: underline;
+}