docs, bpf: Add description about bpftool btf command
docs, bpf: Add description about bpftool btf command

Recently, there have been many updates to bpftool, and the btf command
is one of them. And by using the dump subcommand allows you to check
the debugging information that the BTF has.

This commit adds description to the BPF document about how to use btf
command of bpftool to dump BTF debugging information.

Signed-off-by: Daniel T. Lee <ecc2a1920afd72449920a2db5c036fd6f80996b7@gmail.com>

michi-covalent/cilium,tklauser/cilium,tklauser/cilium,tgraf/cilium,michi-covalent/cilium,cilium/cilium,tgraf/cilium,cilium-team/cilium,tklauser/cilium,michi-covalent/cilium,tklauser/cilium,cilium/cilium,tklauser/cilium,tgraf/cilium,cilium/cilium,tgraf/cilium,cilium/cilium,tgraf/cilium,michi-covalent/cilium,cilium-team/cilium,cilium/cilium,michi-covalent/cilium,tgraf/cilium