[Translations: /de] +/usage, +/quickstart, other improvements

- [x] full translation into german for the complete /usage section and for the quickstart file
- [x] corrected & updated the upgrading section to fix issue #1873 
- [x] corrected spelling mistakes
- [x] global use of `Ghost-Dokumentation` for the `Ghost-Docs`-term
- [x] added [anchors](https://github.com/TryGhost/Ghost/commit/8f47b97b1fb6263172beb5cf5d549e5ad6d9bc81)
- [x] translated the SSL documentation to fix #1855 
- [x] updated theme docs like #1936 
- [x] translated usage-menu