Draws one page's page-number text, vertically centered in the margin strip (same
top+height/2+lineHeight/2 baseline convention as createAnchoredBand's vertical centering)
and horizontally aligned within the page's content width (margins.left to pageWidth-margins.right).
Draws one page's page-number text, vertically centered in the margin strip (same top+height/2+lineHeight/2 baseline convention as createAnchoredBand's vertical centering) and horizontally aligned within the page's content width (margins.left to pageWidth-margins.right).