Update sites/handbook/source/handbook/legal/marketing-collaboration/index.html.md

Why is this change being made?

This change introduces a new workflow for handling reports of third party infringement of GitLab trademarks in browser ads.

There are no currently known instances of this occurring, but this behaviour has happened previously, and we were unable to take action due to our trademark featuring in the supplemental register only.

We've since obtained appropriate trademark registration in the USPTO Principal Register, and can request the take down of these infringement with browser operators, on the basis of this.

Therefore, we have designed this workflow to give guidance to team members on how to handle infringement concerns in browser ads, when they arise in future.

There is detailed discussion on this matter in the issues here and here

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