Create links for 3.1 documentation

The documentation link on the Support page should allow users to select if they would like to view documentation for 3.0 or 3.1. It should also provide links to PDF forms of the documentation. The links will be https://download.phpbb.com/pub/documentation/3.0/olympus_documentation.pdf and https://download.phpbb.com/pub/documentation/3.1/ascraeus_documentation.pdf

The 3.1 documentation section of the site also needs to be created. There is an ascraeus branch off of the phpBB documentation repo.