Add design maintainer areas

Why is this change being made?

Done as part of gitlab-org/gitlab-design#858 (closed).

This is the first MR that attempts to address the lack of clarity on what knowledge is required to become a design maintainer. To do that, I propose that we:

1. Break down project's design maintainership into areas.
2. Start fleshing out the different focuses of review for each project.

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