Adds the correct and finished German translation for the
authentication.md file. Also fixes the index.de.md translation!
This is the complete and improved version of #827
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Co-authored-by: Paul Toffoloni <69189821+ptoffy@users.noreply.github.com>
Co-authored-by: Tim Condon <0xTim@users.noreply.github.com>
Co-authored-by: Mattes Mohr <mail@mattesmohr.de>
Dominik K