Document fallback schedule mechanism

[victorgarcia98]

PR #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.

[Flix6x]

Should this become a general section in https://flexmeasures.readthedocs.io/en/latest/api/introduction.html (e.g. just before Deprecation and sunset) explaining that API endpoints may respond with a status 303 (See Other), with a short example code snippet for clients (similar to what we provide in the Deprecation and sunset section)?

[Flix6x]

Inspiration for possible code snippets:

• feat(schedule): get fallback schedule if redirected to it flexmeasures-client#71
• Retrieve fallback schedules SeitaBV/v2g-liberty#131