fix(docs-infra): add visual signal for external links (#36601)

Clicking on a link may take us to an extenal source, which may lead to user leaving angular.io unintentionally.

Added visual cues on external links so that user knows which links are external and which are intenal to angular.io.

Fixes #17620

PR Close #36601

aio/src/styles/0-base/_typography.scss

@@ -192,3 +192,32 @@ code {
     color: $mediumgray;
   }
 }
+
+.folder-api,
+.folder-cli,
+.folder-docs,
+.folder-guide,
+.folder-start,
+.folder-tutorial {
+  
+  aio-doc-viewer{
+    a {
+      &[href^="http:"]::after,
+      &[href^="https:"]::after {
+        font-family: "Material Icons";
+        content: "open_in_new";
+        margin-left: 2px;
+        position: relative;
+        @include line-height(24);
+        vertical-align: bottom; 
+      }
+    }
+
+    .github-links a {
+      &[href^="http:"]::after,
+      &[href^="https:"]::after {
+        display: none;
+      }
+    }
+  }
+}