Permanently redirect doc to docs

nginx

 # Nginx config file
+server {
+    listen 80;
+    server_name doc.gitlab.com;
+    return 301 $scheme://docs.gitlab.com$request_uri;
+}
+
 server {
 	listen   80; ## listen for ipv4; this line is default and implied
 	root /srv/doc-gitlab-com;
 	index README.html index.html index.htm;
-	server_name doc.gitlab.com;
+	server_name docs.gitlab.com;
 	location / {
 		try_files $uri $uri/ =404;