docs: disallow docs-content for search indexer (#32577)

Co-authored-by: Andrew Seguin <[email protected]>
(cherry picked from commit d77de86)

Author: andrewseguin

docs/src/robots.txt

@@ -1,3 +1,3 @@
 Sitemap: https://material.angular.dev/sitemap.xml
 User-agent: *
-Disallow:
+Disallow: /docs-content/