[DOCS] provide a list of supported versions of Varnish Cache server

It would be nice to have a list of supported Varnish Cache Server versions:

There are currently versions 6(alias stable), 7(alias old) and 8(alias fresh) of Varnish Cache servers available: See: https://varnish-cache.org/releases/index.html

We are using version 6 currently, but there are so much goodies on 7.1+, like build-in on Docker-Image of libvmod-dynamic etc.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information