add links and pluralize Merge-Commit

[GregorDeCillia]

Some small changes in the german version of this document.

• Add the links to the gnu styleguides from the english version
• Use plural "Merge-Commits" rather than singular

[olivierlacan]

@GregorDeCillia Sorry for the late review, do you mind rebasing main? There are several changes and you can now access all links with the same data.links object in any translation.

In this case:

• data.links.gnustyle
• data.links.gnunews

You can remove the Haml variable assignments but other changes from your PR are still needed. Thanks!