kernel-docs: Add new section for Rust learning materials

Include a new section in the Index of Further Kernel Documentation with
resources to learn Rust. Reference it in the Rust index. The resources
are a product of a survey among assistants to the conference Kangrejos'24.

Signed-off-by: Carlos Bilbao <carlos.bilbao.osdev@gmail.com>
Reviewed-by: Dirk Behme <dirk.behme@de.bosch.com>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240922160411.274949-1-carlos.bilbao.osdev@gmail.com