PDF: Visual improvements

[jpmckinney]

See open-contracting/standard#588 (comment)

Once those are made, we can fold this repo into the standard repo, and make generating PDFs part of the build process.

We are not married to wkhtmltopdf as the solution, but it seems to be the best option so far. Other options are Sphinx and Chrome (follow link in open-contracting/standard#588 issue description).

[jpmckinney]

Noting that:

tr {
  page-break-inside: avoid;
}

doesn't seem to fix the issue in the GPA profile of rows breaking. But this should work according to wkhtmltopdf/wkhtmltopdf@5dea253

[jpmckinney]

We should also test/look into options for letting Sphinx build the PDF(s) itself.

See also https://jupyterbook.org/advanced/pdf.html