This shortens some titles and makes them more consistent. It also
removes "JAX" from several titles ("in JAX", "for JAX", "JAX's",
etc.). Since these are JAX docs, that ought to be clear from context.
Building on #21925, this tutorial demonstrates the use of the FFI using
`ffi_call` with a simple example. I don't think this should cover all of
the most advanced use cases, but it should be sufficient for the most
common examples. I think it would be useful to eventually replace the
existing CUDA tutorial, but I'm not sure that it'll get there in the
first draft.
As an added benefit, this also runs a simple test (akin to
`docs/cuda_custom_call`) which actually executes using a tool chain that
open source users would use in practice.
