Add the /help page in robots.txt

The /help page has docs which we don't want to be crawled
as we prefer the docs website instead.

Related https://gitlab.com/gitlab-org/gitlab-ce/issues/44433

public/robots.txt

@@ -20,6 +20,7 @@ Disallow: /projects/new
 Disallow: /groups/new
 Disallow: /groups/*/edit
 Disallow: /users
+Disallow: /help
 
 # Global snippets
 User-Agent: *