[doc] requirements: Add section encouraging DMABUF use for iGPUs

[JJRcop]

Also added link in install_libvirt to the new section.

[gnif]

The wording needs to change a bit here. There is no "copy engine" per say under Linux, it's a DMA transfer, as such it offloads the transfer from the CPU, to the GPUs DMA hardware.

without using an intermediary buffer

It might still use one, it depends on the GPU hardware and driver implementation. iGPUs may even avoid any copy at all as they share system RAM with the GPU and could just use it directly in place (zero copy).

considered necessary for a decent experience.

I would reword this to state it is a requirement.