Add slack and handbook to configure stages.yml

Why is this change being made?

Adding slack and handbook information to Configure stage groups to help people outside the group to know how to ping teams in Issues and find the right associated channel.

See !33021 (closed)

cc/ @dcroft

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what