docs: nginx setup: allow list for csp headers

define which external source to allow when setting csp headers

docs/eln/install_configure/manual_install.mdx

@@ -165,6 +165,8 @@ Here, you are accessing the files `....crt` and `....key`. These files are certi
 
 Further information on how to configure nginx, please refer to tutorials online (e.g. [here](https://docs.nginx.com/nginx/admin-guide/web-server/reverse-proxy/) and [here](https://www.cyberciti.biz/faq/configure-nginx-to-use-only-tls-1-2-and-1-3/)).
 
+NB: When setting the CSP headers, be sure to allow cas.org and doi.org
+
 #### To summarize, you have to make sure that:
 
 - Reverse proxy is setup properly (i.e. DNS resolves to the URL to your server's IP address).