Replace static mut for CORE1_STACK

[jannic]

Both in our docs and in some examples, we suggest using a static mut for allocating memory for core1's stack:

  static mut CORE1_STACK: Stack<SIZE: 4096> = Stack::new();
[...]
    core1.spawn(unsafe { &mut CORE1_STACK.mem }, [...] )

With rust 1.83, this will cause a warning (at least it does with current beta), and with edition 2024 the lint will become deny-by-default. (https://doc.rust-lang.org/nightly/edition-guide/rust-2024/static-mut-references.html)

Therefore, we should suggest something else. Maybe we should also change the signature of multicore::Core::spawn to take a pointer instead of a &'static mut for the stack?

[thejpster]

Yeah we shouldn't use a reference here - a pointer is fine. But not a *const u8 because it might not be sufficiently aligned.

For the global a UnsafeCell with a MaybeUninit makes sense because we don't need to init the stack to anything.