HLT2 line authoring tutorial

To readers

This MR has been merged! The current version of the HLT2 line tutorial can be found here:

https://lhcbdoc.web.cern.ch/lhcbdoc/moore/master/tutorials/hlt2_line.html

Initial description follows.

---

Very much WIP! Will complete and polish this over the course of the hackathon. Feel free to leave comments inline while it's WIP.

https://apearce.web.cern.ch/apearce/moore-docs/tutorials/hlt2_line.html