TRIALS, ARRAIGNMENTS…TELEVISION has acquired an insatiable appetite for this dry legal fare. Watergate, a fascinating five-hour docuseries that begins Sunday (Aug. 7, 9 p.m. ET) on the Discovery Channel, resurrects the granddaddy of them all. What a glorious crew the Senate Watergate hearings assembled in the summer of 1973: the totemic John Mitchell, the original yuppie John Dean, grim hatchetmen H.R. Haldeman and John Ehrlichman, and the phantom of this political soap opera, Richard Nixon. Even at this remove, the proceedings make for spectacular drama.