landlock: Add audit documentation

Because audit is dedicated to the system administrator, create a new
entry in Documentation/admin-guide/LSM .  Extend other Landlock
documentation's pages with this new one.

Extend UAPI with the new log flags.

Extend the guiding principles with logs.

Cc: Günther Noack <gnoack@google.com>
Cc: Paul Moore <paul@paul-moore.com>
Link: https://lore.kernel.org/r/20250320190717.2287696-29-mic@digikod.net


Signed-off-by: Mickaël Salaün <mic@digikod.net>