Docs: Redirect and rename AI doc files

Right now, most of the top-level GitLab Duo pages are under doc/user.

They need to be in subfolders, and have their filenames updated to reflect the GitLab Duo branding.

Specifically, we need to:

• Move these files into subfolders and redirect them
• Update the left nav to match the new location
• Update links in the www- repo
• Update links in the handbook
• Let digital experience know, for example, see this issue
• Let marketing know, for example, see this issue

Maybe we should post this in slack somewhere highly visible, because SAs will also need to know...

To do

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_examples.md	gitlab_duo/use_cases.md * was wrong before
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
project/merge_requests/ai_in_merge_requests.md	project/merge_requests/duo_in_merge_requests.md

See related conversation: !152679 (comment 1933561459)

Also related to: #460106 (closed)