[DE] Add `Rules/Contributor_Code_of_Conduct`

[The-Last-Cookie]

Translation notes

• community leader --> Führungspersönlichkeit or Community-Leiter
• Not sure how the last two external links should be written (i.e. Contributor Covenant and Mozillas code of conduct enforcement ladder)

Self-check

• [x] The changes are tested against the contribution checklist
• [ ] (translations only) The changes are reviewed on GitHub by a fluent speaker