Add NumberDeviceClass.DURATION to documentation of entity number

[kiryph]

Merged PR home-assistant/core#96932 added NumberDeviceClass.DURATION to the entity number (similarly as it exists for the entity sensor).

Afaik, but it missed to update the developer documentation for the entity number

https://developers.home-assistant.io/docs/core/entity/number#available-device-classes

It exists in sensor:

https://developers.home-assistant.io/docs/core/entity/sensor#available-device-classes

[make-all]

It also isn't mentioned in https://www.home-assistant.io/integrations/number/

I always thought this was omitted because it is non-trivial to accept input if the duration is formatted in human readable format, was the change above kept, or is it backed out?