A Study to Learn If Nirmatrelvir (PF-07321332)/ Ritonavir Were Effective and Safe Compared With Placebo in Nonhospitalized Adults With COVID-19 Who Have Low or Standard Risk of Worsening to a Severe Illness. An Interventional Efficacy and Safety, Phase 2/3, Double-Blind, 2-Arm Study to Investigate Orally Administered PF-07321332/Ritonavir Compared With Placebo in Nonhospitalized Symptomatic Adult Participants With COVID-19 Who are at Low Risk of Progressing to Severe Illness.