Clarify design traineeship being optional

Why is this change being made?

Done in the context of gitlab-org/gitlab-design#858 (closed).

The phrasing of design traineeship being optional for someone to become a maintainer wasn't clear, as pointed out by @nudalova (!45728 (comment 320050708)).

This MR also updates some of the Markdown reference links.

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