Add design trainee acceptance limit

Why is this change being made?

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

So that the limits of the design trainee maintainer program are clear and we don't have to answer this question every time.

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