Consider adding a Gradle example project

[jamesnetherton]

E.g some kind of simple CQ project. Mainly for testing that a CQ Gradle app works ok.

We had issues in the past where things ending up not working with Gradle. It'd be good to get an early indication of breakages from the CI build.

[jamesnetherton]

Would need to think how it could work with CI workflows, the release process etc, since everything is currently built around Maven.