For an overview of LabKey URLs, check out this help doc:

https://www.labkey.org/wiki/home/Documentation/page.view?name=url

The routing is based on the module's registered controllers and inner classes of that controller. In the case of the DemoModule, the DemoController is registered as "demo":

    protected void init()
    {
        addController("demo", DemoController.class);
    }

Then the actions within the controller are added based on the class name minus the "Action" part. So in this case: "/labkey/demo/<container>/convertXlsToJson.view"