Add/Change documentation of ISO images for VM installations

[StephanStS]

If ISO images are added to the download page (https://dietpi.com/#downloadinfo), the usage of the ISO images need to be added/changed in the installation description e.g. there: https://dietpi.com/docs/install/#how-to-install-dietpi-proxmox.

See also MichaIng/DietPi-Website#344

[MichaIng]

The idea was to have the detailed manual VM creation steps in an expandable info box, and promote https://github.com/dazeb/proxmox-dietpi-installer instead as default option for the QCOW2 disk images, the installer ISOs as new alternative options. So our instructions are shorter and easier by default.