docs: add LinkedIn banner (1200x628) composing Intel logos + iGPU screenshot

Generated with PIL from existing repo assets (no AI image generation,
just layout). Highlights the three key metrics — ~18 tok/s, $0 token
cost, 94% iGPU utilization — alongside the live Open WebUI screenshot
showing Qwen3-8B running on the Arc 140V.

Suitable as LinkedIn post image and as social card / OG image for the
repo.

Author: cartesiosson