CMake variable and documentation for ALPAKA_DEFAULT_HOST_MEMORY_ALIGNMENT

[bernhardmgruber]

#1686 introduced the macro ALPAKA_DEFAULT_HOST_MEMORY_ALIGNMENT. We should document it properly and consider making it available via cmake.

Originally posted by @bernhardmgruber in https://github.com/alpaka-group/alpaka/issues/1686#issuecomment-1096531865