Resolve "Automatically add category label for groups that contain only one category"

Consider adding documentation to the handbook pages for Triage Operations when creating a new triage policy or triage package:

• https://gitlab.com/gitlab-com/www-gitlab-com/blob/master/source/handbook/engineering/quality/triage-operations/index.html.md

Closes #308 (closed)

I'm adding a new logic into #new_stage_and_group_labels_from_intelligent_inference to infer category if the group only contains 1 category. Since we are only addressing a special case of a group, instead of introducing a new inference process for a category, I'm leaving the method name as it is for now.

/cc @gl-quality/eng-prod