PDF download link

[robredpath]

@stevieflow pointed out that it was a useful feature that the Validator docs can be downloaded as a PDF, but that's currently only available via the RTD flyout. It would be useful if we can find a good place to put the PDF download link into the theme (if possible) so that it's available consistently across the docs without using the flyout.

Maybe we could create a hard-coded section at the bottom of the left-hand nav? I think we'd want some kind of horizontal separation between the ToC and the download section. RTD also creates .epub - I don't think there's necessarily much demand for that but if it looks nicer to have two formats, that's an easy second option. I believe RTD can also create a .zip archive of HTML files for offline reading, which might be a better second option / a third option.