PHILADELPHIA — In September, the Centers for Disease Control and Prevention published guidance for Halloween celebrations: Traditional door-to-door trick-or-treating — or even an outdoor “trunk-or-treat” event — is a high-risk activity, the CDC said.For Ellery Mims, whose Cheltenham Village neighborhood is usually awash with hundreds of costumed candy-seekers on Oct. 31, that guidance was decisive.“Please listen to the CDC!” said Mims, 37. “They’re doctors.”But on neighborhood Facebook groups and the social app Nextdoor, “It’s contentious,” she said. “A lot of people are saying, ‘I’m not doing…