Add complete relevant metadata for articles

[mitchelloharawild]

There are some obscure metadata fields which are used that would simplify generation of articles and improve discovery on other services such as Google Scholar.

For example, metadata$pdf_url is mostly undocumented, but is part of the scholar metadata. When generating the PDF, we should fill in this metadata field, and use it to link to the pdf, while also providing it in the google scholar metadata.