Redo workflow for deploying the PRISMS-PF website

[stvdwtt]

Updating the PRISMS-PF website can be labor-intensive at the moment if you want to update the manual. The manual has its own repo. Doxygen has to be run there and then some html files need to be copied into a directory in the website repo (the ghpages branch of this repo). It would be better to automate this with Travis CI. Then we could automatically rebuild the manual with every update to the website. That way the manual would never get out of date (in terms of Doxygen).