"DigitalOcean supports postGIS?"
Generally speaking, D.O. does not support individual software programs or packages. According to its FAQs, D.O. only provides a "platform developed for developers who need to launch and scale their applications quickly."
To that end, D.O. does provide CentOS images, among others, for developers to build their droplets from. According to the website you provided, PostGIS is available for various operating systems; and CentOS is one of those operating systems.
"how do I install it?"
Install instructions are available @ http://postgis.net/install
