MAINTAINERS: Add the virtio-gpu documentation to the corresponding section

Add virtio-gpu.rst to the corresponding section in MAINTAINERS, so that
the maintainers gets CC:-ed on corresponding patches.

Message-ID: <20231027060808.242442-1-thuth@redhat.com>
Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>

MAINTAINERS

@@ -2483,6 +2483,7 @@ S: Odd Fixes
 F: hw/display/virtio-gpu*
 F: hw/display/virtio-vga.*
 F: include/hw/virtio/virtio-gpu.h
+F: docs/system/devices/virtio-gpu.rst
 
 vhost-user-blk
 M: Raphael Norwitz <raphael.norwitz@nutanix.com>