Adding schedule, and rearranging slide decks and documentation

I moved all of the lessons into their own directory -- there were two instances of each lesson.rst, so I removed one and consolidated them to a single Slides/ directory. I also added winter schedule to front page and a schedule page
where is this content duplicated? maybe it should be a section on the ":get involved" page... but emphasizing that we're newbie-friendly is important and probably should be added elsewhere if you're taking it out here