Add an example demonstrating how to modify the main schedule order

[alice-i-cecile]

How can Bevy's documentation be improved?

Custom schedules can be added by users, and the order of the Main schedule can be changed to meet their needs.

However, the path used to do so is not obvious.

To insert a schedule, mutate the Schedules resource. Then, mutate the MainSchedule resource to set its ordering.

We should add an example demonstrating how to do this, and spot check the related API docs.

[Malax]

I'd like to tackle this if no one else is currently working on it!

[alice-i-cecile]

Awesome, thanks!