Resolve "Current configuration does not work on subpath"

GitLab pages is served under a sub path, https://gitlab.hzdr.de/software.hifis.net. Currently, this is not reflected in the current implementation, which makes the Pages site unusable.

This MR introduces the configuration parameter baseurl that can be used to generate relative urls with Jekyll. All links are made relative throughout the project.

Closes #19 (closed)