OS X Mountain Lion does not include Web Sharing as an option in the Sharing preference pane. Mountain Lion does include the Apache HTTP Server, an open-source web server. For information about enabling and using Apache, see http://httpd.apache.org.
OS X Server includes an interface for managing web sites, along with a dedicated wiki server and many other services such as file sharing, device management, and hosting Time Machine backups. It is available from the Mac App Store.
Third-party applications may also provide a graphical user interface (GUI) for managing web sharing.