Add maturity targets to the package group page

Why is this change being made?

This MR links to the epics that detail our product maturity goals for our respective categories. The aforementioned epics are the SSOT for product maturity goals and are also linked from the Package direction page. https://about.gitlab.com/direction/cicd/#package

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