DoomGate, which is one contender for the title of "earliest community Doom site", was founded by Piotr Kapiszewski at the State University of New York in Buffalo, from which he had recently graduated, in 1994. It was the original go-to site for all things Doom, from newsgroup archives (this was before Deja, let alone Google Groups) to mods of all kinds, to documentation.
DoomGate was originally located at doomgate.cs.buffalo.edu, but at some time prior to December 21, 1997 it moved to its present home on Gamers.org.
Doom Help Service
One thriving part of DoomGate is the Doom Help Service, which mostly gathers together all documentation of a helpful nature, such as the Unofficial Doom Specs. The DHS part of the site has been run by British Doomer Ian Mapleson since 1994; it is still available, but is no longer updated due to no new content being available.