Provide tutorial data and templates

[tfrancart]

Documentation refers to "_data/data.ttl" and "default.html".
It would be nice to provide these files in a complete ready-to-run example, so that people unfamiliar with jekyll (like me) can start playing with.

[tfrancart]

Just found out https://github.com/white-gecko/jekyll-rdf/tree/develop/test/cases/Jekyll
The "template examples" section of the README needs an update.

[white-gecko]

Actually there is a tutorial screencast: https://natanael.arndt.xyz/2018/08/07/jekyll-rdf-tutorial-screencast
and the respective repository: https://github.com/white-gecko/JekyllRDF-Tutorial

Related to #200

[tfrancart]

Many thanks. works fine. A cross reference to this tutorial from README page would help, maybe.