Add getting-started.html link and placeholder

## The Point

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.

## The Screenshot

First of all, @jasonlong, how much do you hate this? It probably needs a different icon for the new button.

![libgit2](https://f.cloud.github.com/assets/39902/1261466/aa74e210-2c2d-11e3-9b55-757a6e530be8.png)

Also, I was thinking of re-using the sidebar to contain a list of tutorials. There'll probably only be one to start with, but maybe 5-10 eventually.

![libgit2-1](https://f.cloud.github.com/assets/39902/1261537/aa5eea4e-2c2f-11e3-8f7d-48cd77b34278.png)

## The To-do List

- [x] Add "Getting Started" link in header
- [ ] Make it look not terrible
- [ ] Refactor to use Jekyll templates
- [ ] Write a tutorial and landing page