Downloading SSL certificate.

Hi,

tl;dr: Is there a way to download the ssl certificate from google pages or SSH into the server to generate a new certificate with certbot? 

Explanation: 

I hosting my (outside domain) website via github pages. I’d like to setup a reverse proxy using nginx from a docker container. I’d have to either regenerate my ssl certificate or download the one that github pages is already using. 

Any suggestions? 

* github pages