docs: Integrate rustdoc generation into htmldocs

Change target `make htmldocs` to combine RST Sphinx and the generation of
Rust documentation, when support is available and .config exists.

Reviewed-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Carlos Bilbao <carlos.bilbao@amd.com>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230718151534.4067460-3-carlos.bilbao@amd.com