add link to manual pdf to the docs until they're fully implemented

[JulianBnder]

since the docs are very bare-bones right now I spent at least 15 minutes looking for a basic usage guide in the docs and the internet so it would help other people to put a link to the manual

I'm not doing a pull request because I think the potential merge conflicts would be more annoying for the maintainers than just adding a link

[johannes-wolf]

The front page contains a link to the manual. I am not sure where you expect the link to the manual.pdf to be.