Add ThOr transition guide for HLT2 lines

With !821 (merged) in, line authors can now in principle start transitioning their selections over to algorithms which use ThOr functors. This MR adds a tutorial on how to migrate an existing selection to ThOr:

http://apearce.web.cern.ch/apearce/moore-docs-thor-configuration/tutorials/thor_transition.html

It also adds a reference guide on ThOr functors themselves:

http://apearce.web.cern.ch/apearce/moore-docs-thor-configuration/selection/thor_functors.html

The latter is is a more technical guide intended to help curious folks understand the details of ThOr and how to compares with LoKi. It also includes LoKi-to-ThOr functor translation tables, with thanks to @vrenaudi and @mferrill:

http://apearce.web.cern.ch/apearce/moore-docs-thor-configuration/selection/thor_functors.html#functor-translation-tables

Once these documents are in place we will be ready to ask line authors to start using ThOr. (At which they will all come up with some vital functors that are missing, but that's another story 😇 )

A couple of to-dos:

• Finish the "How ThOr functors work" section. will leave for later #284
• Migrate the existing HLT2 line tutorial over to ThOr functors.