Need download link for PDF document version

[jfpanisset]

A clear download link for the full PDF version of the document should be on the web site.

I would suggest having a link to the latest version always visible as part of the banner, and under Reference Docs having links for all versions for which we have PDFs in the repo.

[ksallee]

Opened #21 for the first part — an always-visible "Download PDF" link in the banner pointing at the latest guide PDF.

The second part (links to all PDF versions under Reference Docs) is trickier to do cleanly: that tab is generated into data.json from the canonical Google Doc, so the links would need to go into the Google Doc source to survive regeneration. Happy to do the wiring once they're added there.