Docs: Once translations are complete, verify and update links

[nihar1024]

Once textual docs content has been translated, we should update links and URLs to point to the Deutsch version of websites where possible.

For eg.: https://plan4better.de/en/goat/ --> https://plan4better.de/goat/

This should only be done for pages under the i18n/de/docusaurus-plugin-content-docs/current folder.