I have actived this RenderDoc, but there is no icon in the window.
You need to install renderdoc manually on your pc from here https://renderdoc.org. After you have installed you can open unreal and you should see the render doc icon on the top right corner of the viewport.
I think you need to set the path to installed directory in ProjectSettings
PS v5.1 work
In UE5.3.2-release, it works by following steps.