0.13: One-Shot system Improvements

[Trashtalk217]

For the release notes (#891).

@Nathan-Fenner, I'd love it if you could proofread the text (if you have the time) and let me know if everything is correct.
I have kept it purposefully short, as I don't see it as a particularly big change, just some QOL.

Also, I don't quite know who to credit. I'm currently referring to changes done in

• Add 'World::run_system_with_input' function + allow World::run_system to get system output bevy#10380 (significant work)
• Exclusive systems can now be used for one-shot systems bevy#11560 (just a documentation change)
• Allow registering boxed systems bevy#10378 (already in since 12.1, but people may not know about it so why not)

and I've only really given credit to Nathan Fenner so far, as that amounts to the lion's share of the work.

[alice-i-cecile]

I think Nathan Fenner is the right call for who to credit there: the other PRs are pretty incidental.

[Nathan-Fenner]

I left a few small notes.

From the technical side, I think it's cool that run_system(id) is now just the same as run_system_with_,input(id, ()) - bevy's generic types are quite magically expressive. It is a testament to Bevy's leveraging of Rust's type system just now easy this was to implement.

[Nathan-Fenner]

The type of id is slightly different now, it's a SystemId<i32, i32>, (though plain SystemId still works and now means SystemId<(), ()>) which might be worth mentioning- possibly also making the in/out types different in the example to clearly show which is which.

[Trashtalk217]

For the sake of brevity, I think I'll leave it out of the main post, but I like the idea of adding type annotations and maybe a comment to the code example. I'll do that later.

[TrialDragon]

From a writer's standpoint this is good in grammar, readability, and being understandable other than what Nathan has already brought up.