rust: alloc: add doctest for `ArrayLayout::new()`

rust: alloc: add doctest for `ArrayLayout::new()`

Add a rustdoc example and Kunit test to the `ArrayLayout` struct's
`ArrayLayout::new()` function.

This patch depends on the first patch in this series in order for the
KUnit test to compile.

Suggested-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://github.com/Rust-for-Linux/linux/issues/1131
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Jimmy Ostler <jtostler1@gmail.com>
Link: https://lore.kernel.org/r/f1564da5bcaa6be87aee312767a1d1694a03d1b7.1734674670.git.jtostler1@gmail.com
[ Added periods to example comments. Reworded title. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>