docs: move DCO and CoC to CONTRIBUTING

[MikeMcC399]

Description

Move the following sections:

• Developer's Certificate of Origin 1.1
• Code of Conduct

from the GOVERNANCE document into the CONTRIBUTING document, aligning with the nodejs/node > CONTRIBUTING document as reference model and for better discoverability.

Motivation and Context

The CONTRIBUTING document does not contain information regarding the DCO or the Code of Conduct. This is instead included in the GOVERNANCE document, where it may not be discovered by contributors.

Taking the nodejs/node > CONTRIBUTING document as reference model, this is the place that rules concerning contributing are placed directly, not the GOVERNANCE document.

The nodejs/node-gyp > CONTRIBUTING document also follows this model, as a further example.

Testing Details

Types of changes

• Documentation
• Version change (Update, remove or add more Node.js versions)
• Variant change (Update, remove or add more variants, or versions of variants)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Other (none of the above)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING.md document.
• All new and existing tests passed.

[MikeMcC399]

Preview

• https://github.com/MikeMcC399/docker-node/blob/contributing-dco-coc/CONTRIBUTING.md#code-of-conduct
• https://github.com/MikeMcC399/docker-node/blob/contributing-dco-coc/CONTRIBUTING.md#developers-certificate-of-origin-11

[nschonni]

I took a quick skim of https://developercertificate.org/ and I think the text is all up-to-date