Update SRC process docs

[tallclair]

I did a review pass on our existing docs. I didn't address all issues, but made a number of improvements including:

• Update oncall workflow with latest triage methodology
• Update security response process with more realistic timelines & processes
• Deleted the "Severity Thresholds". AFAIK no one was using this, and it didn't map to our actual severity evaluation.
• Renamed PSC to SRC in the onboarding doc

Fixes https://github.com/kubernetes/committee-security-response/issues/146

Addresses many comments from https://github.com/kubernetes/committee-security-response/pull/139

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: tallclair

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[enj]

/lgtm