Migrate away from wkhtmltopdf library (to weasyprint)

It seems `wkhtmltopdf`, which we use for generating PDFs from HTML documents, is [archived](https://github.com/wkhtmltopdf/wkhtmltopdf) and no [longer receives any updates](https://wkhtmltopdf.org/status.html#summary).

We should migrate to a different library. The creator of the old library recommends [`weasyprint`](https://doc.courtbouillon.org/weasyprint/stable/first_steps.html#). It seems it also can do HTML to PDF conversions, so hopefully it won't be a difficult migration. If you know a different/better option that is also fine.