Where prologues are concerned, it all comes down to one thing:
It all depends on what the prologue contains. You mention a major plot point, but you also say:
Building a website for my world with minor character sketches, short stories, mythologies, etc and additional supplemental is one possibility.
This seems to suggest that the majority of the content is in fact worldbuilding. That could be a problem.
A prologue is part of the story. If you told the story without the prologue, things would be missing and the story would be incomplete. A prologue differs from the rest of the story in that it is introductory. That is, it introduces the story, usually the main conflict or characters. This doesn't mean it happens years before the events of the novel.
Some prologues happen years after the novel opens, or even in the middle of the novel, to show you how things are going to turn out. The rest of the novel is about figuring out how things got to where they are in the prologue. A difference in time is a side effect, not a determining factor of, a prologue.
A common type of prologue is the backstory. Backstory gets a bad rep, but there's nothing wrong with it if handled correctly. Backstory should be necessary information that would otherwise never come to light in the novel. The key word there is 'necessary'.
Consider Harry Potter. The first chapter is essentially a prologue. (Note that, even though it is a prologue, Rowling made it chapter one anyway. It works either way.) While at first it might seem that it imparts nothing necessary, it does show the reader that magic and wizards exist in the world. Without that information, it could seem quite far fetched when magic starts appearing in the other-wise painfully normal world of the Dursleys.
Worldbuilding is pure backstory. The difference here is that your story can do without it. Does this small part of your world change the story? No? Then it's pure worldbuilding.
Worldbuilding should not be put in a prologue. Remember: chances are the reader is there for a story, not your world. If you open with an enthusiastic description of a faraway place he's never heard of, he's likely to close the book. Especially in this day and age. Prologues should stick to the necessary information.
That being said, worldbuilding is not something to be avoided at all costs. It has its uses. Worldbuilding is the process by which you create the setting, and setting can have a profound effect on the story. It can set the mood. It can alter reality to move the plot forward, or even keep it from moving at all. It can have profound effects on your characters, often even causing them to perform actions they otherwise wouldn't.
Worldbuilding can definitely be a part of your story. The question is: does the reader need to know it first?
Let me go back to Harry Potter. The murder of Harry's parents by Voldemort is backstory. While not worldbuilding, it will illustrate my example. It has a profound effect on the story. So why didn't Rowling open with it?
There are two reasons. Firstly, the reader did not need to know it immediately. This is of course largely due to Harry not knowing it himself, but even if he had, I doubt Rowling would have opened with it. Why?
Because of the second reason: it's something that can be incorporated into later chapters. As major as Voldemort murdering Harry's parents was, it was easy to incorporate it later on, once it became relevant. The same can be said about virtually all relevant worldbuilding.
Do take note when I say 'relevant'. A seemingly unimportant detail of worldbuilding can still contribute to the overall feeling of the setting in a scene, and thus contribute to the actions of a character (or whatever the setting is designed to do in that instance).
Knowing these differences, it's easy to know what is and isn't a prologue. A prologue is something that has these characteristics:
- Is necessary to the story
- Must be known by the reader before anything else
You can of course simply make the prologue chapter one like Rowling did. There is literally no difference. If you know readers/publishers will cringe when they see 'prologue' in the table of contents, this might be a good option.
The only reason not to do this would be if the prologue is so separated from the story that it just doesn't fit. In that case, a prologue would be better. Don't sacrifice your story for anything. Even the publisher. Explain why you need a prologue. If they can't understand that, you might want to find a different publisher.
You can also draw some conclusions about worldbuilding:
- Necessary worldbuilding contributes to setting and atmosphere. It is included when necessary, not before.
- Unnecessary world-building adds nothing to either atmosphere, setting, or story. You make this easily accessible to fanatics of your work, but easily ignored by common readers just after a good story. This type of world-building is rarely found in the book, but is almost always found on your website.
Hopefully knowing these differences will help you in your writing endeavors. And congratulations on completing your first draft!
Determine first if the information is relevant to the story. If it is, determine if it needs to be known by the reader before anything else. If the answer to both questions is yes, you can make it a prologue. However - unless doing so would make it seem overly out of place - also consider simply naming it chapter one anyway. This could be the way to go if you fear readers/publishers will throw it out due to seeing 'prologue' in the table of contents. There is no difference between a prologue called a prologue, and one called 'chapter one'.
If the information is irrelevant to the story, do not include it. If the information is relevant to the story, but only in a setting/atmospheric context, include it only when you need to establish that setting/atmosphere. This should only be done to either give the story a particular feel, or cause the characters to do something they otherwise wouldn't.
Finally, if the information is relevant, but the reader does not need to know it first, include it later on in the story as it becomes relevant.