Enable doxygen verification.

[francocipollone]

Pairs with ToyotaResearchInstitute/ament_cmake_doxygen#6
Related to maliput/maliput_infrastructure#227

One doxygen warning :

test 2
    Start 2: maliput_dragway_doxygen_warnings

2: Test command: /bin/bash "/home/franco/maliput_ws/install/ament_cmake_doxygen/share/ament_cmake_doxygen/cmake/../tools/check_doc_warns.sh" "/home/franco/maliput_ws/build/maliput_dragway/ament_cmake_doxygen/maliput_dragway/warnings.log"
2: Test timeout computed to be: 9.99988e+06
2: Error. The following warnings were found in /home/franco/maliput_ws/build/maliput_dragway/ament_cmake_doxygen/maliput_dragway/warnings.log.
2: /home/franco/maliput_ws/src/maliput_dragway/include/maliput_dragway/road_geometry.h:52: warning: The following parameters of maliput::dragway::RoadGeometry::RoadGeometry(const api::RoadGeometryId &id, int num_lanes, double length, double lane_width, double shoulder_width, double maximum_height, double linear_tolerance, double angular_tolerance, const math::Vector3 &inertial_to_backend_frame_translation) are not documented:
2:   parameter 'inertial_to_backend_frame_translation'
2/5 Test #2: maliput_dragway_doxygen_warnings ...***Failed    0.00 sec

[agalbachicar]

LGTM