Refactor layout to enable adding CI and Documentation

[robertmaynard]

The original layout of rapids-cmake mixes support directories ( testing ) with directories holding rapids-cmake function ( cpm, export ). As we introduce more support functionality such as documentation, or try to provide test support functions this mixture will make it harder for users to discover functionality.

This PR moves all rapids-cmake functions into the rapids-cmake folder. This will make it easier for users/developers to know the intention of each folder in rapids-cmake.

This shouldn't break backwards compat, as the root CMakeLists.txt has been updated so that the following pattern still works:

include(FetchContent)
FetchContent_Declare(
  rapids-cmake
  GIT_REPOSITORY git@github.com:rapidsai/rapids-cmake.git
  GIT_TAG        origin/main
  )
FetchContent_MakeAvailable(rapids-cmake)
include(rapids-cmake)
include(rapids-cpm)
include(rapids-cuda)
include(rapids-export)
include(rapids-find)