docs: added expire static files nginx directive for better caching

docs/admin/system_admin/nginx/nginx-config-example.rst

@@ -110,6 +110,7 b' Use the following example to configure N'
         #    gzip_types  text/css text/javascript text/xml text/plain text/x-component application/javascript application/json application/xml application/rss+xml font/truetype font/opentype application/vnd.ms-fontobject image/svg+xml;
         #    gzip_vary on;
         #    gzip_disable     "msie6";
+        #    expires 60d;
         #    alias /path/to/.rccontrol/community-1/static;
         #    alias /path/to/.rccontrol/enterprise-1/static;
         # }

docs/admin/system_admin/tuning/tuning-scale-horizontally-cluster.rst

@@ -263,6 +263,7 b' 6) Configure `Nginx`_ as reverse proxy o'
             gzip_types  text/css text/javascript text/xml text/plain text/x-component application/javascript application/json application/xml application/rss+xml font/truetype font/opentype application/vnd.ms-fontobject image/svg+xml;
             gzip_vary on;
             gzip_disable     "msie6";
+            expires 60d;
             #alias /home/rcdev/.rccontrol/community-1/static;
             alias /home/rcdev/.rccontrol/enterprise-1/static;
         }