Create handbook page for Third-party Trademark Usage Guidelines

Why is this change being made?

As per https://gitlab.com/gitlab-org/gitlab/-/issues/323772, a need has been identified for guidelines covering the use of third-party trademarks in the GitLab product. These guidelines have been discussed and drafted in https://gitlab.com/gitlab-org/gitlab/-/issues/323772 and this google doc. This MR:

1. Amends the GitLab handbook page to add a reference to the guidelines; and
2. Creates a new page under handbook/legal/policies/third-party-trademark-usage-guidelines/ to host the guidelines.

• One outstanding point remains before this can be merged: As per the final bullet point in the Dos and Dont's section, we need to identify the directory into which third-party trademarks should be saved when added to the product.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files. (this requirement has been removed pending identification of a new DRI for the handbook)