Add valid examples to `std::ptr::{read,write}_unaligned`

[Centril]

With #62323 the only example (that had UB and was thus invalid) in std::ptr::read_unaligned and std::ptr::write_unaligned is removed.

We should add a valid example of using the aforementioned functions.

cc @rust-lang/wg-unsafe-code-guidelines

This issue has been assigned to @Freyskeyd via this comment.

[RalfJung]

Slices are one context in which one could conceivably create an unaligned raw pointer:

fn read_usize(x: &[u8]) -> usize {
  assert!(x.len() >= mem::size_of::<usize>());
  let ptr = x.as_ptr() as *const usize;
  unsafe { ptr.read_unaligned() }
}

Thanks to @Shnatsel on Zulip.

The fix for this bug thus is to add an example like this, and a similar one for writes, to the docs for read_unaligned and write_unaligned. The docs should also explain why using just read or write is UB.

[Freyskeyd]

Hello!

I would like to work on this one :)

[Freyskeyd]

@rustbot claim

[RalfJung]

Fixed by #62493.