Improve encapsulation for commands and add docs

[JoJoJet]

Objective

Several of our built-in Command types are too public:

• GetOrSpawn is public, even though it only makes sense to call it from within Commands::get_or_spawn.
• Remove and RemoveResource contain public PhantomData marker fields.

Solution

Remove GetOrSpawn and use an anonymous command. Make the marker fields private.

---

Migration Guide

The Command types Remove and RemoveResource may no longer be constructed manually.

// Before:
commands.add(Remove::<T> {
    entity: id,
    phantom: PhantomData,
});

// After:
commands.add(Remove::<T>::new(id));

// Before:
commands.add(RemoveResource::<T> { phantom: PhantomData });

// After:
commands.add(RemoveResource::<T>::new());

The command type GetOrSpawn has been removed. It was not possible to use this type outside of bevy_ecs.

[james7132]

Thanks for the docs!