ggml-virtgpu: add backend documentation (#19354)

ggml-virtgpu: add backend documentation (#19354)

* ggml-virtgpu: add backend documentation

Assisted-by-AI: Claude Code

* CODEOWNERS: add /docs/backend/GGML-VirtGPU/ -> kpouget

* README: add the link to docs/backend/GGML-VirtGPU/ggml-virt.md

* docs/ggml-virt: add link to testing + configuration

* Revert "CODEOWNERS: add /docs/backend/GGML-VirtGPU/ -> kpouget"

This reverts commit 8ece8e72e24d305f308505c08ebb75804546374e.

* drop the ggml- prefix

* s/ggerganov/ggml-org

* Relocate VirtGPU.md

* reorganize the text

* turn turn the ascii diagram into a mermaid

* README.md: update the link to the main doc