Document fallback schedule mechanism

[victorgarcia98]

PR https://github.com/FlexMeasures/flexmeasures/pull/846 introduces a new way to handle fallback schedulers: schedule a fallback job on failure of the main schedule. Moreover, exposes this information via the API by returning the status code 303 on request of the original job with the location of the new resource. This behavior needs to be described in the documentation.