Remove all mentions of sheepdog from the docs

As sheepdog is no longer actively maintained according to one of the
developers, it is planed to remove it for the upcomming version 6 of
Proxmox VE.

This removes mentions of sheepdog from the docs - at least from the
parts which are not auto-generated and thus must wait for PVE 6.0
when also the plugin will be removed.

Signed-off-by: Christian Ebner <c.ebner@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>