Cosmetic fixes to getting started notebook. (#4062)

* Cosmetic fixes to getting started notebook.

There were some problems with the version on the PyMC3 docs site.
Fixed some warnings.  Tidied up some markup (some of which didn't seem to process properly on gh pages), and added a note explaining about pm.Deterministic

* Improve date plot.

Very ugly way to keep xticklabels from crashing into each other. Not sure if this is the right thing, or if ArviZ should be fixed to automatically format date-times better.

* Add example of pm.Deterministic

Thanks to @AlexAndorra.

Author: rpgoldman