tools: bpftool: update references to other man pages in documentation

Update references to other bpftool man pages at the bottom of each
manual page. Also reference the "bpf(2)" and "bpf-helpers(7)" man pages.

References are sorted by number of man section, then by
"prog-and-map-go-first", the other pages in alphabetical order.

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>