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/mainloop.cuh

@@ -7,8 +7,8 @@
 
 #include <type_traits>
 
-#include "../../common/mma.cuh"
-#include "../common.h"
+#include "common/mma.cuh"
+#include "fp8/common.h"
 #include "load.cuh"
 #include "policy.cuh"