Add section on tutorials and how-tos

[rmglennon]

@rfriberg and I have a start to the content in a branch. I need to figure out where to put it and push some changes to add it to the build process.

[rmglennon]

The section on general developer guides has been created, but it needs a home so people can find it without a link.

Most likely, this is adding a new page with links to all tutorials.

https://github.com/mapzen/mapzen-docs-generator/pull/226