Elevate "Groups" category to "Subgroups" and a marketing category

Why is this change being made?

The capability of nesting groups is a unique one to GitLab. We should mention this in our categories, and elevate this category to a marketing category so that users are more aware of it.

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