Translate authentication.md to german (#889)

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 <[email protected]>
Co-authored-by: Tim Condon <[email protected]>
Co-authored-by: Mattes Mohr <[email protected]>