Rename Growth Section to Growth Sub department

Why is this change being made?

Growth is a sub-department. See https://about.gitlab.com/handbook/engineering/

This updates only the Growth Engineering page (not team.yml, categories.yml, product pages, or even individual group pages)

Added a table of Groups with links and group labels as an easy way to access individual pages (this was the real reason I made the change).

Using Growth sub department except in the title (Growth Sub-department)

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

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