docs(drag-drop): add docs and live example for delay input

[crisbeto]

Adds some docs and a live example for the cdkDragStartDelay input. Follow-up from #14732.

Note: targeting major, because the PR that introduces the functionality was merged into the next major branch.

[jelbourn]

LGTM

[ngbot]

I see that you just added the pr: merge ready label, but the following checks are still failing:
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