bpf, docs: Add introduction for use in the ISA Internet Draft

bpf, docs: Add introduction for use in the ISA Internet Draft

The proposed intro paragraph text is derived from the first paragraph
of the IETF BPF WG charter at https://datatracker.ietf.org/wg/bpf/about/

Signed-off-by: Dave Thaler <dthaler1968@gmail.com>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20240422190942.24658-1-dthaler1968@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>