bpf, docs: Document BPF_MAP_TYPE_ARRAY

Add documentation for the BPF_MAP_TYPE_ARRAY including kernel version
introduced, usage and examples. Also document BPF_MAP_TYPE_PERCPU_ARRAY
which is similar.

Co-developed-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: Dave Tucker <dave@dtucker.co.uk>
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Maryam Tahhan <mtahhan@redhat.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Link: https://lore.kernel.org/bpf/20221109174604.31673-2-donald.hunter@gmail.com