Move style guide into a separate contributors category

[rossbar]

Identified by @rgommers in #81 (comment) - it would be an improvement to keep the pedagogical material (i.e. tutorials, how-tos, explanations, etc) separate from info about contributing, e.g. the tutorial style guide.

This would involve a minor reorganization to move the tutorial style guide out of the main toctree and reference it from a "contributing" section instead.