The WPI package brings MediaWiki to the world of Web Platform Installer, Web App Gallery and WebMatrix.
The current version bundles MediaWiki 1.27.0 with the WindowsAzureStorage extension. It can be found here.
The sourcecode for the packages up to version 1.25.1 can be found in Diffusion. Newer versions are located at Github.
In your Web Platform Installer, just select MediaWiki from the application list and click install. You have to fill in a few parameters and the installation process will start automatically.