[Kereth]

Good documentation is important in any MUD, and it can be handy for this documentation to be visible on your website. In particular, important things to have on your website are those involved in character planning/generation. People who come to your page for the first time are, hopefully, getting ready to play, or at least considering it. If character generation is more than choosing a name, as it is with many, but not all MUDs, information on how to go about that generation, details about classes, races, statistics, etc, should be readily accessible on the page. Without that, it's all-too-easy to get into a game for the first time, be faced with a list of choices of some kind, and have no idea what to choose or where to go, only to end up doing something really ignorant and ruining your first character attempt ("Wait, you mean I should have rolled wisdom for a spellblade? I thought it was purely an int thing, and what do you mean my unicorn doesn't have any hands to wield the sword!?") Similarly, you may find it easier to organize certain kinds of information for quick-reference on a website, where they can be navigated with hyperlinks, rather than within the MUD itself, where manually typing out helpfiles is the norm. A website probably shouldn't be a substitute for good in-game documentation of anything, but it certainly can help as a secondary source for it.

Aside from that, I agree with what NW said here. It varies based on your MUD. If there are puzzles to be solved, dungeons to be explored, quests to be worked through, and half the fun is solving/exploring/working through them, don't spoil that for everyone, but if it's a pure PK MUD where everyone's just there to twink out, minmax, and take on the world, then by all means, give them the study material to know how to do it.

Oh, and a wiki is not a substitute for a webpage, though it may be a supplement.
-Kereth