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/tests/attn_paged_test.cu

@@ -1,5 +1,5 @@
 // Compile:
-//   nvcc -I csrc -arch=sm_89 -O3 --use_fast_math --ptxas-options=-O3 \
+//   nvcc -I csrc/kernels -arch=sm_89 -O3 --use_fast_math --ptxas-options=-O3 \
 //        --extra-device-vectorization -Xcompiler -fopenmp \
 //        csrc/tests/attn_paged_test.cu \
 //        -o /tmp/test_paged && /tmp/test_paged
@@ -7,7 +7,7 @@
 #include <cstring>
 #include <vector>
 #include "test_utils.cuh"
-#include "../kernels/attention/dispatchers.cuh"
+#include "attention/dispatchers.cuh"
 
 using namespace astrai::attention;