Print targets should get better links in pelican generated site

[oscarlevin]

Currently the @deploy-dir getting set in the manifest tells pretext deploy to build a static landing page for multiple projects. If you use this for a print target, the landing page will create a link to that directory. It would be much better to create a link to the actual pdf.

[oscarlevin]

It seems like if the user specifies a output-filename then the link points to the correct thing. I suspect this is just an oversight in the pelican generation, using none instead of the default, perhaps.