Update free-programming-books-de.md

Added missing Pro Git eBook with the german translation.
Fix two index links
Fix index link in ru file
Merge pull request #530 from Robalni/fix-ja-links

Fix two index links in japanese file
Merge pull request #529 from romankl/patch-1

Update free-programming-books-de.md
Web2Py thank to @mdipierro #526
Merge pull request #531 from Robalni/fix-ru-link

Fix index link in ru file