ALEXIS OHANIAN/ INSTAGRAM Serena Williams‘ cheering section just gained a new member. The tennis champ has welcomed her first child — a daughter! — with her fiancé, Reddit co-founder Alexis Ohanian, multiple outlets report. Baby girl arrived Friday, Sept. 1 at St. Mary’s Medical Center in West Palm Beach, Florida, weighing in at 6 lbs., 13 oz., according …