rust: list: Add an example for `ListLinksSelfPtr` usage

It appears that the support for `ListLinksSelfPtr` is dead code at the
moment [1]. Although some tests were added at [2] for impl `ListItem`
using `ListLinksSelfPtr` field, still we could use more examples
demonstrating and testing the usage of `ListLinksSelfPtr`. Hence add an
example similar to `ListLinks` usage.

The example is mostly based on Alice's usage in binder driver [3].

Link: https://lore.kernel.org/rust-for-linux/20250719183649.596051-1-ojeda@kernel.org/ [1]
Link: https://lore.kernel.org/rust-for-linux/20250709-list-no-offset-v4-5-a429e75840a9@gmail.com/ [2]
Link: https://lore.kernel.org/rust-for-linux/20231101-rust-binder-v1-4-08ba9197f637@google.com/ [3]
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
[ Fixed typo. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

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