doc: toolchains: Update custom toolchain CMake reference file names

This commit updates the CMake reference file names for custom
toolchains.

The contents of `cmake/generic_toolchain.cmake` and
`cmake/target_toolchain.cmake` were moved
`cmake/modules/FindHostTools.cmake` and
`cmake/modules/FindTargetTools.cmake`, respectively.

Refer to the commit 8d2998d4 and
7131d02f for more details.

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>