Add @katrinleinweber as German proofreader

What does this MR do?

See title ;-) As per the proofreader docu: I previously translated OpenSource.guide, and contributed EN->DE translations to Mailpile & AntennaPod.

cc @mhah, the other German proofreader: Hello :-)

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer