Update performance regression policy

[davehunt]

Description

The performance regression policy at https://www.mozilla.org/en-US/about/governance/policies/regressions/ is out of date. We need to update the contact details, add a section for sheriffing criteria, and a few other small updates. The updated content is here: https://docs.google.com/document/d/1v8Wyl_Bjp1uJxRxstjVPHSRLiQxHVdxa6oYAfomqEdI/edit#

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Success Criteria

• Performance regression policy content is updated

[alexgibson]

@davehunt pushed this to a demo so you can review before it goes out. Let me know if this looks ok? Thanks

https://www-demo2.allizom.org/en-US/about/governance/policies/regressions/

[davehunt]

Looks good, thanks @alexgibson