validate single email per github handle in sigs.yaml

[BenTheElder]

We should be able to validate that we don't wind up with multiple different emails somehow for contributors with entries for multiple SIGs/WGs

follow-up to #8138

[BenTheElder]

uhhhh
/committee steering
/sig contributor-experience

[BenTheElder]

... this is a bit messy to add. We have a validation phase but the output is buried and the make verify scripting is wired only to show a generated diff rather than validation errors.

Really it would be better if we didn't have to duplicate people definitions and refactored them out to be defined once and referred to by github handle within each of the references in groups, but that's a breaking change and I'm hesitant about what may break.

You could imagine using the github handle as a unique key (since github already enforces that) to refer to a single shared map of people by name / employer / email / ...