docs(docs-infra): fix a11y of external links (#43725)

prevent screen readers from falsely announcing "open_in_new" since they may not actually open new tabs/windows but simply send the current page to a domain outside angular.io

Fixes #43512

PR Close #43725

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

@@ -213,7 +213,7 @@ code {
           position: relative;
 
           &::after {
-            content: "open_in_new";
+            content: "\e89e"; // codepoint for "open_in_new"
             font-family: "Material Icons";
             position: absolute;
             right: 0;