Change `ament_cmake_doxygen` dep to `doc_depend` to avoid ros build to fail

[francocipollone]

• [x] maliput
• [x] maliput_py https://github.com/maliput/maliput_py/pull/61
• [x] maliput_dragway https://github.com/maliput/maliput_dragway/pull/70
• [x] maliput_drake (No ament_cmake_doxygen dependency)
• [x] maliput_multilane https://github.com/maliput/maliput_multilane/pull/87
• [x] maliput_malidrive
• [x] maliput_integration https://github.com/maliput/maliput_integration/pull/118
• [x] maliput_integration_tests
• [x] maliput_object
• [x] maliput_object_py https://github.com/maliput/maliput_object_py/pull/3
• [ ] delphyne
• [ ] delphyne_gui
• [ ] delphyne_demos