File change and redirects for GitLab Duo docs

• Please check this box if this contribution uses AI-generated content as outlined in the GitLab DCO & CLA.

What does this MR do?

Moves these files into subfolders and redirects them:

Old file	New location/name
ai_data_usage.md	gitlab_duo/data_usage.md
ai_experiments.md	gitlab_duo/experiments.md
ai_features_enable.md	gitlab_duo/turn_on_off.md
ai_features.md	gitlab_duo/index.md
gitlab_duo_chat_enable.md	gitlab_duo_chat/turn_on_off.md
gitlab_duo_chat_examples.md	gitlab_duo_chat/examples.md
gitlab_duo_troubleshooting.md	gitlab_duo_chat/troubleshooting.md
gitlab_duo_chat.md	gitlab_duo_chat/index.md
gitlab_duo_examples.md	gitlab_duo/use_cases.md
project/merge_requests/ai_in_merge_requests.md	project/merge_requests/duo_in_merge_requests.md

Related issues

• Docs: Redirect and rename AI doc files (#465362 - closed)
• Draft: Update directory items in nav fot GitLab... (gitlab-docs!4830 - closed)

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