Add getRootResource method to avoid multiple routes

This method avoids to write multiple repetitive routes for resources like
`GET     /api-docs.json/pet            controllers.ApiHelpController.getResource(path = "/pet")`
`GET     /api-docs.json/cat            controllers.ApiHelpController.getResource(path = "/cat")`
Thanks to it, adding this route will handle all controllers if the name follows the convention
`GET     /api-docs.json/*path            controllers.ApiHelpController.getRootResource(path: String)`