The Cofoundry admin panel uses a modular system that let's you slot in your own sections into the UI. An admin module simply points to a route so technically you can use any front-end framework you like to implement your section, but in most cases you'll want to use the Cofoundry admin framework which is implemented in Angular.js.
This area is still in development and will likely change in a future update. In particular the admin framework will likely be migrated away from Angular.js 1.x to a more modern framework.
Documentation and support will be provided once the admin module and plugin system has stabalized (likely post-1.0). If you want to go ahead anyway the best reference is the source code for Cofoundry.Web.Admin which uses the same modular system