Write real docs for SystemSet

[alice-i-cecile]

Objective

SystemSets are surprisingly rich and nuanced, but are extremely poorly documented.

Fixes #19536.

Solution

Explain the basic concept of system sets, how to create them, and give some opinionated advice about their more advanced functionality.

Follow-up

I'd like proper module level docs on system ordering that I can link to here, but they don't exist. Punting to follow-up!

[viridia]

Show an example of adding a system to a system set. (Or link to a place which explains it).

[viridia]

Nit: I would probably want to steer novices in the direction of using app.add_system(MySystem.in_set(MySet)) rather than a custom schedule, which seems like an advanced topic. However, maybe that's intentional.

[alice-i-cecile]

Nit: I would probably want to steer novices in the direction of using app.add_system(MySystem.in_set(MySet)) rather than a custom schedule, which seems like an advanced topic. However, maybe that's intentional.

Me too! Unfortunately, this is in bevy_ecs, which does not rely on bevy_app, so I can't pull in the required types.

[viridia]

Ugh, I have run into that problem as well. Makes it hard to tell a coherent story if you can't talk about the purposes for which a given thing exists.

[theotherphil]

One typo, but otherwise LGTM!