refactor: root kernel includes at csrc/kernels

- add the kernels directory to CMake target include paths and drop all ../-relative includes in kernel sources
- reference shared primitives as common/*.cuh and the fp8 type header as fp8/common.h
- update standalone test nvcc commands in file headers and cuda_kernels.md to -I csrc/kernels

csrc/kernels/fp8/gemm.cuh

@@ -11,8 +11,8 @@
 #include <cuda_runtime.h>
 #include <type_traits>
 
-#include "../common/cp_async.cuh"
 #include "common.h"
+#include "common/cp_async.cuh"
 #include "gemm/epilogue.cuh"
 #include "gemm/load.cuh"
 #include "gemm/mainloop.cuh"