Extend Support for CUDA and HIP kernel invocations

This MR extends the support for CUDA and HIP kernel invocations and API reflection in pystencils-sfg, reflecting the concepts introduced in pystencils!449 (merged).

• Move GPU-related stuff into new composer mix-in SfgGpuComposer
• Replace cuda_invoke() by gpu_invoke()
• Implement three separate signatures of gpu_invoke(), depending on the kernel indexing scheme selected by the user
• Replace extensions.gpu by lang.gpu
• Add explanations on GPU kernels to user manual
• Fix some errors and clarify some interface descriptions in the user manual

This MR will already be using Target.HIP and therefore will have to wait for pystencils!458 (merged).

tests/generator_scripts/index.yaml

@@ -94,7 +94,7 @@ MdSpanLbStreaming:
 
 CudaKernels:
   sfg-args:
-    file-extensions: ["cuh", "cu"]
+    file-extensions: ["hpp", "cu"]
   compile:
     cxx: nvcc
     cxx-flags: