New gh-pages website

Based on PR #533 I decided to enhance the existing website at the gh-pages branch.

The current status can be seen at http://flagbug.com/Mahapps.Metro.Website/

I've also ported the existing documentation to the Controls page and linked everything together.

I created a new PR instead of pushing to the existing, because I rebased everything on the gh-pages branch.

- [ ] Fix config.yml before merging