Bugfix: Missing logo in OcSidebar

After to changes in the formerly referenced structure of the ownCloud.com website, no logo was displayed in the sidebar anymore. 
Due to license changes in this repository, it was decided not to include an ownCloud-branded logo anymore, 
so this change adds a placeholder svg file in `assets/example/`. 

https://github.com/owncloud/owncloud-design-system/pull/1090
