Mention the ACME API URI somewhere in the documentation

[osirisinferi]

When I was searching for the actual ACME API URI for Let's Encrypt, I couldn't find it anywhere in the documentation: not in the "getting started" or "how it works", nowhere.

Sure, Certbot is the recommended ACME client and defaults to LE, but what if that changes in the future? You never know! Or if a user has a different client without Let's Encrypt support build in and needs to input the API URI manually?

I think it would be nice to at least mention the API URI in the "Getting started" page. :)