bpf, doc: add bpf list as secondary entry to maintainers file

We recently created a bpf@vger.kernel.org list (https://lore.kernel.org/bpf/

)
for BPF related discussions, originally in context of BPF track at LSF/MM
for topic discussions. It's *optional* but *desirable* to keep it in Cc for
BPF related kernel/loader/llvm/tooling threads, meaning also infrastructure
like llvm that sits on top of kernel but is crucial to BPF. In any case,
netdev with it's bpf delegate is *as-is* today primary list for patches, so
nothing changes in the workflow. Main purpose is to have some more awareness
for the bpf@vger.kernel.org list that folks can Cc for BPF specific topics.

Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>