[Admin manual] Document occ commands with maintenance mode suggestions

[miaulalala]

Some commands are available and safe during maintenance mode, others, due to Events being dispatched / apps not being loaded/ etc might not be.

Extend the section on occ commands inhttps://docs.nextcloud.com/server/latest/admin_manual/configuration_server/occ_command.html to document the specific behavior on whether a command is safe or unsafe.

[kesselb]

#10208
#10244