Add download link to PDF in HTML output

[tomschr]

Problem Description

When you read the HTML output in our documentation and you want to download the PDF, this is very inconvenient: you need to go "up", find the index page, and select the respective PDF.

Expected Behaviour

It would be nice to have a direct download link in the HTML output.

The tricky part could be how to get the file name of the PDF and where to link to. The file name could be passed through daps. For the link, it is probably useful to have some relative link (like ../<BASE>_color_<LANG>.pdf).