Add a general CODE_OF_CONDUCT and CONTRIBUTING file for stackitcloud - #2

This pull request adds clear guidelines for contributing to the project and establishes a code of conduct for all participants. The new documentation sets expectations for behavior, outlines contribution processes, and introduces requirements for commit sign-offs to ensure legal compliance.

Community standards and behavior:

• Added a CODE_OF_CONDUCT.md file, adopting the Contributor Covenant v2.1, which sets expectations for respectful, inclusive participation and details enforcement procedures for unacceptable behavior.

Contribution process and requirements:

• Added a comprehensive CONTRIBUTING.md file that explains how to report bugs, suggest enhancements, and submit pull requests, including steps for claiming issues and interacting with maintainers.
• Introduced guidelines for commit signing using the Developer Certificate of Origin (DCO), with instructions for signing off commits and amending unsigned commits.
• Clarified the distinction between contributors and maintainers, and referenced the new code of conduct as a requirement for all participants.

Signed-off-by: Andy Riexinger andreas_horst.riexinger@digits.schwarz