rust: alloc: kvec: add doc example for as_slice method

Add a practical usage example to the documentation of KVec::as_slice()
showing how to:
Create a new KVec.
Push elements into it.
Convert to a slice via as_slice().

Co-developed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Hui Zhu <zhuhui@kylinos.cn>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Kunwu Chan <chentao@kylinos.cn>
Reviewed-by: David Gow <davidgow@google.com>
Link: https://lore.kernel.org/r/4e7f396f38ed8a780f863384bfc3d7de135ef3ea.1753929369.git.zhuhui@kylinos.cn


Signed-off-by: Danilo Krummrich <dakr@kernel.org>