Created OpenTrails Course Content

v1 of the content for the OpenTrails course. Later iterations will see these pages evolve into a full-throated resources for publishing OpenTrails, much like the recent Open Data Resources that was published.

This pull request includes:
- Updated trail spec home page to more prominently advertise the OpenTrails Course, and to link to new content.
- Created home page for OpenTrails Course, describing outcomes, pre-requisites, time commitment, support available
- Created a page for each week of the OpenTrails course, providing an overview of how an agency prepares to publish and maintain open data for trails.