[keycloak-dev] Examples overview README.md

I've added 'examples/README.md' with a brief overview over all the examples. This replaces 'examples/demo-template/README.md.overview', which only mentioned the demo. Now I'll add instructions to 'unconfigured-demo/README.md' on how to configure it with an existing Keycloak server. I'll make sure the instructions work with either a local or external server. It would be good if the screencast follows the same order as this README.md so they can be used together.