Add help page for run configurations

Currently, the help button on the edit run configuration dialog will take you to e.g.
`https://www.jetbrains.com/help/rider/2018.1/?reference.dialogs.rundebug.UNITY_DEBUG_RUN_CONFIGURATION`, based on the ID of the run configuration. This will 404, as jetbrains.com doesn't know anything about this plugin. We need to specify our own documentation page (e.g. the wiki, or maybe a Jekyll pages site) in the plugin itself.

There is a new API in 183 that will help here.