docs/bpf: Document BPF_MAP_TYPE_LPM_TRIE map

Add documentation for BPF_MAP_TYPE_LPM_TRIE including kernel
BPF helper usage, userspace usage and examples.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221101114542.24481-2-donald.hunter@gmail.com