docs/bpf: Document BPF ARRAY_OF_MAPS and HASH_OF_MAPS

Add documentation for the ARRAY_OF_MAPS and HASH_OF_MAPS map types,
including usage and examples.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20221108102215.47297-1-donald.hunter@gmail.com