docs: rust: Add instructions for the Rust kselftest

Add section describing how to build and run the Rust kselftest.

Signed-off-by: Laura Nao <laura.nao@collabora.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Reviewed-by: Valentin Obst <kernel@valentinobst.de>
Link: https://lore.kernel.org/r/20240405153841.320459-1-laura.nao@collabora.com


[ Formatted paths as inline code literals. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>