Enforcing DCO in the contributing guide

Problem

We do not know external contributors and want to make sure we do not include contributions to which we do not have copyright for.

Goal

Make sure that external contributors only make contributions if they can legally do so (right to submit it under the open source license).

ACs

• DCO is discussed with FITKO's legal department (not part of story point estimation)
• DCO is enforced for all contributors (in template repo and all other repos)
• Contributing.md is updated

Notes/ resources

• German version of the Linux Foundations's DCO: https://opencode.de/de/wissen/rechtssichere-nutzung/cla-dco-handreichung#3.-Template:-Developers-Certificate-of-Origin-(DCO)
• https://developercertificate.org/
• Requirements uIDs: OZG_10; Zapuk_10; Zapuk_12; Zapuk_13

Who needs to be involved / informed

• reviewers:
• involved:
• informed: