demo: add an example project using Hibernate Data Repositories

[jknack]

• See https://docs.jboss.org/hibernate/orm/7.0/repositories/html_single/Hibernate_Data_Repositories.html
• Complement with demo: add an example project showing the full power of the new open-api JavaDoc support #3747

[jknack]

https://github.com/jooby-project/library-demo