Remove the `gitlab_duo_usage_menu_item` feature flag

What does this MR do and why?

Removes the gitlab_duo_usage_menu_item feature flag, which makes the Settings > GitLab Duo page available in top-level groups that support it.

Also documents this new page instead of the Settings > Usage quotas > GitLab Duo tab.

Changelog: added EE: true

MR acceptance checklist

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Screenshots or screen recordings

How to set up and validate locally

Please refer to !156951 (merged)

Related to #464019 (closed)