Handle page numbering when merging

[KojoEnch]

When merging several PDFs in one with the MergePdfBuilder, it would be really useful to have proper numbering of pages in the merged file.

Use case: generate a PDF with a different header for each part, while maintaining correct page numbering in the footer.

Current solution: generate partial PDFs and then merge them.

The issue is that page numbering in the footer restarts at 1 for each part, and it's not possible to pass a footer into the merge.

[KojoEnch]

I opened an issue in gotenberg/gotenberg.