Cats-effect 3 IO docs are missing

[lukestephenson]

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cats-effect 2 has some great documentation around the IO datatype at https://typelevel.org/cats-effect/docs/2.x/datatypes/io.

These docs do not appear to exist for cats-effect version 3.

It would be great to add them.

Given how core this is to cats-effect, I suspect I've somehow missed where they are obviously located, but I can't find them.