landlock: Extend documentation for kernel support

Extend the kernel support section with one subsection for build time
configuration and another for boot time configuration.

Extend the boot time subsection with a concrete example.

Update the journalctl command to include the boot option.

Cc: Kees Cook <keescook@chromium.org>
Reviewed-by: Günther Noack <gnoack@google.com>
Link: https://lore.kernel.org/r/20240227110550.3702236-1-mic@digikod.net


[mic: Improve wording as suggested by Günther]
Signed-off-by: Mickaël Salaün <mic@digikod.net>