Missing recommented Redis configuration for manual setup

[robert-scheck]

As of writing, admin/manual_core.rst does unfortunately not recommend any default Redis configuration. However practically, https://community.grommunio.com/d/426-redis-at-grommunio-documentation-for-debian11/2 works more or less when adapted for the specific Linux distribution. Please note that especially RHEL does not support instanced systemd.units in Redis by default (applies most likely also to Debian when looking carefully to the mentioned link above), like maybe other Linux distributions (only openSUSE?) do.