Add zmartins as a backend trainee maintainer for GitLab main repository

Why is this change being made?

Step required to become a trainee maintainer as described in https://about.gitlab.com/handbook/engineering/workflow/code-review/#trainee-maintainer.

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