bpf, docs: Better scale maintenance of BPF subsystem

The BPF subsystem consists of a large number of pieces. There is not a
single person that understands it all. Yet reviews are crucially important
for the BPF community to provide productive quality feedback to contributors
in a timely manner and therefore to ultimately expand the number of active
developers in the community.

So far, the BPF community had a two-stage review system, that is, a weekly
rotation among 7 developers (Alexei, Daniel, Andrii, Martin, Song, Yonghong,
John) as a first-level review of all inbound patches accompanied by a BPF CI
system which runs the in-tree BPF selftests to check for regressions for
every new patch, and then, a final check by Alexei, Daniel, Andrii to apply
the patches to either bpf or bpf-next trees.

This system worked well for the last ~3.5 years, but clearly reaches its
limits these days as it does not scale enough. Especially, as we also need
to allow enough room for every developer to contribute patches themselves,
integrate with their day to day job, and in particular avoid burnout. We
want to better scale both horizontally and vertically going forward.

On the horizontal scale, we are adding more developers (KP, Stan, Hao, Jiri)
to the overall core reviewer team, thus growing to 11 people in total. The
weekly rotation for the horizontal oncall reviewer is shortened to 1/2 week
(Mo - Wed and Thur - Fri). Instead of just patches, the coverage however
extends also generally to triage and reply to mailing list traffic (e.g. RFCs,
questions, etc).

On the vertical scale, there is clearly a need for deep expertise areas to
assign dedicated maintainer/reviewer teams that are responsible for code
reviews and help with design of individual building blocks. To some degree
we have been doing this implicitly, but the point is to formalize the teams
and commitment.

There is an overlap between areas and boundaries are intentionally grey. These
additional entries provide a guidance on who has to look at the patches. The
patch series which span multiple areas will be looked at by multiple people.
The vertical review with areas of deep expertise are bundled at the same time
with the horizontal side.

This patch cleans up a bit the BPF entries, adds mentioned developers to
the horizontal scale and creates new sub-entries with teams for developers
committing to the above outlined vertical scale. Also, pw.git tools we use
for BPF tree maintenance have been updated with a new pw-schedule script to
semi-automate vertical oncall review rotation.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <martin.lau@linux.dev>
Acked-by: Song Liu <song@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Acked-by: Mykola Lysenko <mykolal@fb.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: KP Singh <kpsingh@kernel.org>
Acked-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Hao Luo <haoluo@google.com>
Acked-by: Quentin Monnet <quentin@isovalent.com>
Link: https://git.kernel.org/pub/scm/linux/kernel/git/dborkman/pw.git
Link: https://lore.kernel.org/bpf/5bdc73e7f5a087299589944fa074563cdf2c2c1a.1656353995.git.daniel@iogearbox.net