[Governance Enhancement] Need to update Code of Conduct, refer to upsteam file instead of maintaining copy

[kevin-wangzefeng]

Issue Description

1. The current Volcano community code-of-conduct.md was directly copied from the CNCF/Foundation one, and subsequent updates have not been synced.

2. There's no explicit instruction about who will be handling the issue if any unacceptable behavior occurs.

Proposed Changes

1. Use a reference link to https://github.com/cncf/foundation/blob/main/code-of-conduct.md instead of maintaining a copy of the content.
2. Add the violation reporting email address to CODE_OF_CONDUCT.md to clarify how community members can report instances of unacceptable behavior. For example:
       > Instances of abusive, harassing, or otherwise unacceptable behavior may be reported by contacting the project team at {email address here}.