Drones are nowadays used in many scenarios, from air surveillance and search-and-rescue missions to agriculture and cinematography. Drones' miniaturisation led to light nano-drones that fit the palm of a hand. Despite some downsides like the battery life of the order of minutes and limited computational power of the onboard microcontroller unit (MCU) belonging to the sub-100 mW power envelope, such systems can navigate in narrow spaces and being harmless around humans can be used in many scenarios. Over the last decade, autonomous drone racing (ADR) competitions have fostered research and provided an opportunity for scientists to create cutting-edge perception and control algorithms meant to operate directly onboard the drones.