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.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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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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[yongruilin]

/retest