Use exemplar projects in Tekton example (dev, staging, prod)

[a-roberts]

With the improvements made at #65, we're still referencing contributor's repositories - worse still, the example repositories don't conform to our GitOps structure. So we end up Docker building them when surely that isn't what we want. Ideally, you'd just need

https://github.com/rhd-gitops-example/gitops-example-dev
https://github.com/rhd-gitops-example/gitops-example-staging
and a third repository, https://github.com/rhd-gitops-example/gitops-example-prod.

Under this issue, modify the example to use these pipelines successfully - building and pushing the code to Dockerhub for example).

We currently reference

• https://github.com/a-roberts/gitops-repo-testing
• https://github.com/akihikokuroda/promote-demo
• https://github.com/akihikokuroda/gitops-test

While these are fine to be forked for testing, let's have something more official.

[mnuttall]

I think this has now been done under #70.