what is the earliest known documentation of atlantis?
Around 355 BCE, Plato wrote two Socratic dialogues, Timaios and Kritias. These were explicitly fiction, but as part of the dialogue Plato has a character relate a supposedly ancient oral tradition received from an Egyptian priest by his grandfather (for a Greek "an Egyptian told me" is a bit like "thus spake wise Confucius"):
Nine thousand years have elapsed since she founded yours, and eight thousand since she founded ours, as our annals record. Many laws exist among us which are the counterpart of yours as they were in the olden time. I will briefly describe them to you, and you shall read the account of them at your leisure in the sacred registers. In the first place, there was a caste of priests among the ancient Athenians, and another of artisans; also castes of shepherds, hunters, and husbandmen, and lastly of warriors, who, like the warriors of Egypt, were separated from the rest, and carried shields and spears, a custom which the goddess first taught you, and then the Asiatics, and we among Asiatics first received from her. Observe again, what care the law took in the pursuit of wisdom, searching out the deep things of the world, and applying them to the use of man. The spot of earth which the goddess chose had the best of climates, and produced the wisest men; in no other was she herself, the philosopher and warrior goddess, so likely to have votaries. And there you dwelt as became the children of the gods, excelling all men in virtue, and many famous actions are recorded of you. The most famous of them all was the overthrow of the island of Atlantis. This great island lay over against the Pillars of Heracles, in extent greater than Libya and Asia put together, and was the passage to other islands and to a great ocean of which the Mediterranean sea was only the harbour; and within the Pillars the empire of Atlantis reached in Europe to Tyrrhenia and in Libya to Egypt. This mighty power was arrayed against Egypt and Hellas and all the countries bordering on the Mediterranean. Then your city did bravely, and won renown over the whole earth. For at the peril of her own existence, and when the other Hellenes had deserted her, she repelled the invader, and of her own accord gave liberty to all the nations within the Pillars. A little while afterwards there were great earthquakes and floods, and your warrior race all sank into the earth; and the great island of Atlantis also disappeared in the sea. This is the explanation of the shallows which are found in that part of the Atlantic ocean.'
Such was the tale, Socrates, which Critias heard from Solon; and I noticed when listening to you yesterday, how close the resemblance was between your city and citizens and the ancient Athenian State.
It is important to emphasize that this earliest depiction of Atlantis was very plainly an allegorical device for a perfect city; Timaios was written as a sequel to Plato's better-known dialogue The Republic, with the same cast of characters, and was a work of philosophy and rhetoric rather than any kind of history. The fictional Atlantis was expanded upon in the following dialogue; I won't quote the whole thing but you can read an English translation here.
Plato never finished that dialogue, so whatever else he had to say about Atlantis is hypothetical. Subsequent authors commenting on Plato and his dialogues touch on Atlantis, but not to add new information - Plato remains the sole source, and even commentators like Poseidonios aren't clear if the island was real or fictional. The idea that the island might be historical reality wasn't strongly put forward until Proklos (Proclus Lycaeus) the Neoplatonist several hundred years later (5th century CE).
Some folks heavily into the idea that Atlantis was a historical reality have proffered bits of ancient texts referring to islands or peoples which they claim as support for the historicity of Plato's Atlantis, but very tellingly none of them either use the name that Plato gave or correspond in detail with his story as it has come down to us.
The myth of Atlantis spread from there, sometimes taken as fiction, allegory, or historical reality.