OBS books moved to github.com

The OBS books, which are accessable on these sides are hosted now at github.com. These books differ from the wiki pages by providing only reviewed content and are the first place where we document new implemented features.

We decided to move to github.com to allow easier contribution and still have some review process via the pull request mechanism of github.com. You can find the repository in the offical openSUSE name space: https://github.com/openSUSE/open-build-service-documentation. Check the README file on how to generate the documentation.