mold.data.frame() should be documented with default_xy_engine()

[DavisVaughan]

Do the same with the other engines

mold() is more of a stub that pushes you through to the other engine specific mold() pages.

That is where documentation for x and y is done, which makes the documentation cleaner to read.

[DavisVaughan]

This means that new_default_xy_engine and the other new_default_*() constructors should be moved to their own page (they share lots of arguments anyways, and if we left them in there with mold.data.frame it would just be confusing)

[github-actions]

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