Migrate AssertJ Assertions Generator documentation

The documentation for the AssertJ Assertions Generator and the corresponding Maven plugin should be migrated from https://github.com/joel-costigliola/assertj.

Once done, the `assertj-generator-gradle-plugin` [README](https://github.com/assertj/assertj-generator-gradle-plugin/blob/main/README.md) should be updated.