doc: Update staging branch list and clarify publishing rules

[yongruilin]

This PR updates the staging branch list and clarify publishing rules in the contributor directory.

Which issue(s) this PR fixes:

Fixes #

[k8s-ci-robot]

Hi @yongruilin. Thanks for your PR.

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[johnbelamaric]

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: johnbelamaric, yongruilin

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