Update documentation for 2.8 release of slapd-cli

[davidcoutadeur]

We must update the new name from openldap-initscript to slapd-cli See https://ltb-project.org/documentation, and especially:

https://ltb-project.org/documentation/openldap-initscript
https://ltb-project.org/documentation/nagios-plugins/check_ldap_slurpd_status
https://ltb-project.org/documentation/nagios-plugins/check_berkeleydb_locks

We must also provide more howtos for the new targets of slapd-cli.

There is also some clean to do

[davidcoutadeur]

Documentation has basically been migrated here: https://ltb-project-documentation.readthedocs.io/en/latest/

It will be updated gradually. This task should not be linked to 2.8 release of slapd-cli

[davidcoutadeur]

Documentation has been migrated, and slapd-cli has been updated everywhere.