explicitly document github moderation policies

[BenTheElder]

Split out from kubernetes/steering#291 (comment)

TLDR: We have moderation policies well defined for slack / discuss / ... but we don't seem to specify them for github (even though we have been taking reasonable moderation behavior under the purview of @kubernetes/owners )

@kubernetes/owners: Can we please clearly write down the policies you all have been enforcing in a location that explicitly scopes github? We should cross reference them from communication/moderation.md, but I think maybe ./github-management might be the right place to document it.

/sig contributor-experience
/committee steering
/area github-management

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/lifecycle stale

[Priyankasaggu11929]

/remove-lifecycle stale