Add example maven/gradle projects that use the annotation processor 

Both for documentation and verification purposes.

The Maven compiler plugin only supports `annotationProcessorPaths` from 3.5:
https://maven.apache.org/plugins/maven-compiler-plugin/compile-mojo.html

TODO: add this section to the `example-maven-project-simple`:

```
          <compilerArgs>
            <arg>-Aproject=${groupId}/${artifactId}</arg>
          </compilerArgs>
```