content: add external icon to play button

The play button now shows an icon to indicate that clicking that button
will take the user to an external site.

Change-Id: I92c0f13ee22f1f04ae12196d0d656f8ec088ffcb
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/259214
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jamal Carvalho <jamal@golang.org>
diff --git a/content/static/css/stylesheet.css b/content/static/css/stylesheet.css
index 94f668a..00d4f4e 100644
--- a/content/static/css/stylesheet.css
+++ b/content/static/css/stylesheet.css
@@ -1311,6 +1311,18 @@
   margin-right: 0.4rem;
   padding-right: 0.5rem;
 }
+.Documentation-examplePlayButton::after {
+  background-image: url(/static/img/icon-launch.svg);
+  background-repeat: no-repeat;
+  background-size: 0.875rem 1.25rem;
+  content: '';
+  display: inline-block;
+  height: 1rem;
+  left: 0.3125rem;
+  position: relative;
+  top: 0.125rem;
+  width: 1rem;
+}
 .Documentation-filesList {
   column-count: 3;
   column-width: 12.5rem;
diff --git a/content/static/css/unit_files.css b/content/static/css/unit_files.css
index cee5dd1..5c8ad61 100644
--- a/content/static/css/unit_files.css
+++ b/content/static/css/unit_files.css
@@ -43,4 +43,4 @@
 .UnitFiles-file::before {
   content: url(/static/img/pkg-icon-file_16x12.svg);
   margin-right: 0.75rem;
-}
\ No newline at end of file
+}