docs(drag-drop): add doc section and live example for customizing the placeholder

[crisbeto]

Adds a paragraph about the cdkDragPlaceholder and sets up a live example.

Fixes#13765.

[jimitndiaye]

@crisbeto Might I suggest expanding the example to be similar to the ToDo connected sorting example with two lists (one of which is not a cdkDropList)? The reason I bring that up is that because the cdkDragPlaceholder is defined on the ckdDrag rather than the cdkDropList you'd need to have a second cdkDragPlaceholder on the drag items of the second list even though the placeholder will be shown in the first list. This is not obvious behavior (at least to me) and probably needs to be emphasized.

[jelbourn]

LGTM

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