Add AI usage guidelines for UX

Why is this change being made?

This merge request adds comprehensive guidance for how UX teams should use AI tools in their work.

This content is required for one of the modules of the AI Up-Skill for product designers: 1 - AI Experiences in My Work < AI Up-Skill (gitlab-org/gitlab-design#2923)

• The new documentation covers when and how to appropriately use AI for tasks like user research, content writing, and prototyping, while emphasizing that AI should assist rather than replace human judgment and user-centered thinking.
• It includes best practices for data safety, prompt writing, bias checking, and transparency about AI usage.
• The guide also specifies which AI tools are approved for use at GitLab (like Claude, Dovetail, and GitLab Duo) and warns against using AI for critical decisions or as a substitute for real user feedback.
• Additionally, it links to this new AI usage guide from related pages.

🤖 MR description drafted by GitLab Duo. MR changes based on research of Claude, ChatGPT, and Perplexity. All content reviewed and edited by @pedroms.

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Commits

• Add AI usage guidelines for UX

Also update related documentation to link to the new page.

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