Create a demo repository with a sample YAML

Demo repository includes:

• Jenkinsfile.yaml, similar to https://github.com/jenkinsci/simple-pull-request-job-plugin/pull/6
• Some demo projects, which is being REALLY built by this Jenkinsfile.yaml (with some reporting)
• The demo publishes Injected Test results, Findbugs and publishes Plugin JAR as an artifact
• Idea: Build SPRB plugin with Jenkinsfile.yaml, put the file directly to the repository