Modify Getting Started documentation to be in line with symfony docs and have YAML examples.

I've been setting up Sonata myself and had to slow down a bit in step 4, the admin service creation, because the examples were in XML not YAML.  The symfony docs use YAML so I figured I'd see if I could add them in.  I also updated the DependencyInjection code to be directly copy-pastable with the [symfony cookbook DI examples](http://symfony.com/doc/master/cookbook/bundles/extension.html#using-the-load-method) (mainly `use` the `Loader` class and reference `Loader\YamlFileLoader` instead of `use`ing `Loader\YamlFileLoader`).

I also updated the PHP code to say which files they come from and include the function signatures that they belong to.  I'm not sure what the standard is in terms of if to include the class that it came from also (i.e., should I put `class YourNSAdminBundleExtension extends Extension` at the top of the code fragment?).

Let me know if I didn't format this pull request correctly.  I know some groups like everything squashed to 1 commit, I couldn't find a contributions document quickly.