link the unreproducible package in the docs section

https://github.com/bmwiedemann/theunreproduciblepackage/

should be linked prominently on our webpage, very probably in the docs section.

currently it's just mentioned in:

_blog/posts/155.md
_blog/posts/131.md
_blog/posts/172.md
_blog/posts/156.md
_blog/posts/133.md
_blog/posts/168.md
_blog/posts/189.md
_blog/posts/185.md
_events/paris2018/report.md
_events/berlin2017/ReproducibleSummitIIIEventDocumentation.md
_reports/2019-07.md
_reports/2020-04.md