add tutorial on explainable reasoning with categorical variables

[rfl-urbaniak]

Goal: prepare an easy entry point for people who are not too much into the weeds.

[rfl-urbaniak]

@eb8680 @SamWitty

Here's a revised version of the tutorial.

• using the dev version of Pyro added model renderings
• inspired by initial confusion in a discussion with Poorva, I added a bit more explanation in the motivations section
• added more context for the but for condition
• added an explanation of what antecedents, witnesses, and consequents are
• moved a detailed discussion of auxiliary variables in the trace till the end
• toned down the discussion of actual causality as such, sending the reader to a different notebook
• tried to provide multiple explanations for queries, starting with more informal ones, then moving to a bit more formal ones, then the code

[SamWitty]

@rfl-urbaniak , this PR is labeled as ready for review but the lint CI fails. Please either fix the linter or change the tag to WIP.