Add getting-started.html link and placeholder

This site needs some tutorials and such. I've tried to do some of this with [my blog](http://ben.straub.cc/blog/categories/libgit2/), but it'd be better if there was introductory material on the libgit2 site itself. More official like.

- [x] Figure out navigation structure
- [x] Make it look not terrible
- [x] Refactor to use Jekyll templates
- [x] Write an example
- [x] Write a guide
- [x] Flesh out the docs landing page