MAINTAINERS: add docs entry to AMDGPU

To make sure maintainers of amdgpu drivers are aware of any changes
 in their documentation, add its entry to MAINTAINERS.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Tales Lelo da Aparecida <tales.aparecida@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

MAINTAINERS

@@ -15928,6 +15928,7 @@ S:	Supported
 T:	git https://gitlab.freedesktop.org/agd5f/linux.git
 B:	https://gitlab.freedesktop.org/drm/amd/-/issues
 C:	irc://irc.oftc.net/radeon
+F:	Documentation/gpu/amdgpu/
 F:	drivers/gpu/drm/amd/
 F:	drivers/gpu/drm/radeon/
 F:	include/uapi/drm/amdgpu_drm.h