Switch to simpler promote-demo example instead of referring to Aki's

[a-roberts]

I'll do this one, going with a simple Go app instead. Things take a minute with the simple Pipeline atm and 16 minutes with the Buildah one I've got in a PR.

With the example I'll propose, it's one minute for the Buildah Pipeline.

[a-roberts]

Code's at https://github.com/a-roberts/promote-demo

@mnuttall @bigkevmcd lemme know if you want me to create a repository under this org and push that code there, then I can update our readme to reference it at https://github.com/rhd-gitops-example/promote-demo. Or you can create the repo and I'll PR into it