Move implode() to array section and/or cross-ref.

[Girgias]

This is from note 129797 whose writer does not have GitHub:

This will probably be deleted, but it feels like this function would be better suited under the array function section since the input is an array.

This is based on the key input object for the function: an array.

If we base categorization of built-in functions on output, then a bunch of array functions could probably move to other sections of the documentation, too.

Alternatively, these could be cross-listed or a redirect could be added to one page or the other.

From manual page: https://php.net/function.implode

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