docs: Update community files

[alessfg]

Proposed changes

This PR updates the various community files to bring them up to the latest standards:

• Updates the Code of Conduct to v2.1.
• Replaces issue templates with issue forms.
• Replaces mentions of the community Slack with mentions of the community forum.
• Adds a CODEOWNERS file.

Checklist

Before creating a PR, run through this checklist and mark each as complete:

• I have read the CONTRIBUTING document
• I have run ./update.sh and ensured all entrypoint/Dockerfile template changes have been applied to the relevant image entrypoint scripts & Dockerfiles
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation

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