services: certbot: Associate one certificate with several domains.

* doc/guix.texi (Certificate Services): Document <certificate-configuration>,
the change from domains to certificates and the fact that their path is now
derived from their name.
* gnu/services/certbot.scm (<certificate-configuration>): Add and export it.
(certbot-configuration, certbot-command, certbot-activation,
certbot-nginx-server-configurations, certbot-service-type): Replace 'domains'
with 'certificates'.
(certbot-nginx-server-configurations): Use only one nginx-server-configuration
and use all certificate domains as the server-name.