* How-to -> Tutorials; Learn -> Resources (URL is the same)

* Reorder navbar, change of names
Details
-------
Quickstart = API quickstart (no navbar link, linked big blue button + tutorials)
Learn -> Books+Videos
Quickstart(old) -> About PyMC3

* Added syntax highlighting to landing page code snippet
* Added see also section at the end of About PyMC3
* Classified Tutorials into Basic and How-Tos

* Incorporate @junpenglao 's comments

* Final tweaks to doc restructure
Backwards compatible `make serve`, no dangling "Other", "intro.rst" -> "history"