landlock: Document network support

Describe network access rules for TCP sockets. Add network access
example in the tutorial. Add kernel configuration support for network.

Signed-off-by: Konstantin Meskhidze <konstantin.meskhidze@huawei.com>
Link: https://lore.kernel.org/r/20231026014751.414649-13-konstantin.meskhidze@huawei.com


[mic: Update date, and do light cosmetic changes]
Signed-off-by: Mickaël Salaün <mic@digikod.net>