MEXICO CITY — The number of journalists killed as a result of their work more than doubled in 2020, an international media watchdog group said on Tuesday, with armed conflict and gang violence making Mexico and Afghanistan among the deadliest countries for reporters globally. At least 30 journalists were killed worldwide this year, according to the watchdog group, the Committee to Protect Journalists, with 21 of those killings carried out as a direct response to the reporters’ work, compared to 10 in 2019. While the total number of killings rose in 2020, the number of deaths related to conflict fell to its lowest level since 2000, according to the C.P.J., with waning violence in the Middle East and fewer reporters traveling because of the coronavirus pandemic. 