rust: list: add `impl_list_item!` examples

rust: list: add `impl_list_item!` examples

There's a comprehensive example in `rust/kernel/list.rs` but it doesn't
exercise the `using ListLinksSelfPtr` variant nor the generic cases. Add
that here. Generalize `impl_has_list_links_self_ptr` to handle nested
fields in the same manner as `impl_has_list_links`.

Tested-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Link: https://lore.kernel.org/r/20250709-list-no-offset-v4-5-a429e75840a9@gmail.com
[ Fixed Rust < 1.82 build by enabling the `offset_of_nested`
  feature. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>