I had the same thought. Regardless, the actual instructions to install the DNS plugins are in the install page. https://certbot.eff.org/lets-encrypt/centosrhel7-nginx
Basically, replace certbot in the install command with the DNS plugin you need i.e. certbot-dns-digitalocean for Digitalocean.
That said, this worked better than the instructions listed creating new wildcard certs (Note: this one renews an existing cert called example.org):
sudo certbot certonly --cert-name example.org \
--dns-digitalocean \
--dns-digitalocean-credentials ~/digitalocean.ini \
--server https://acme-v02.api.letsencrypt.org/directory \
-d "*.example.org" -d example.org