Add examples of how we do stable counterparts

Why is this change being made?

See https://gitlab.com/gitlab-com/gl-security/appsec/appsec-team/-/issues/415+

@ameyadarshan and I were chatting about being Stable Counterparts, and how the handbook page has some small recommendations, but some team members (or others in the industry using our handbook) might benefit from having a more structured approach documented. That will at least provide a starting point people can take and modify.

We also discussed how being SC can seem like a daunting task, especially when we imagine all the things others might be doing that we're not doing (leading to imposter syndrome, etc).

We got a bunch of responses, and to keep things Efficient I reckon we just paste them straight into the handbook.

Author Checklist

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