Update aliasnbpages examples

Update examples at https://mpdf.github.io/reference/mpdf-functions/aliasnbpages.html
since mPDF 6 the placeholders are only supported in the headers/footers. Not in the body.

https://mpdf.github.io/what-else-can-i-do/replaceable-aliases.html has been updated.


see also https://github.com/mpdf/mpdf/issues/267