Add Custom SubApp example

[radiantgurl]

Objective

Fixes some issues related to #20975

error[E0433]: failed to resolve: use of unresolved module or unlinked crate `bevy_app`
 --> examples/app/custom_subapp.rs:3:17
  |
3 | #[derive(Clone, AppLabel)]
  |                 ^^^^^^^^ use of unresolved module or unlinked crate `bevy_app`
  |
  = help: if you wanted to use a crate named `bevy_app`, use `cargo add bevy_app` to add it to your `Cargo.toml`
  = note: this error originates in the derive macro `AppLabel` (in Nightly builds, run with -Z macro-backtrace for more info)

Solution

Modify Cargo.toml and also add a new example.

Testing

cargo run --example custom_subapp

Works as intended.

[radiantgurl]

May i ask why is it marked as contentious? It is just an example addition and adding AppLabel to the crate prelude