Default assumes that Doxygen is installed and is available via PATH. Change executableName if that's not the case.
Default location for the Doxygen config file in src/doxygen/doxygen.config. Change with doxygenConf.
You can get Doxygen to generate a default config file with doxygen-report:create-config.
When creating the reports, the plugin will modify the config file and insert the following parameters at the top:
PROJECT_NAME = "${project.artifactId}"
PROJECT_NUMBER = "${project.version}"
OUTPUT_DIRECTORY = "${doxygen-report.outputDirectory}"
INPUT = "${doxygen-report.input}"
RECURSIVE = YES
They can be overridden by re-defining them in the doxygen config file.