Add Duo Governance settings page and card

What does this MR do and why?

Implements the Governance card section on the GitLab Duo settings page (/-/settings/gitlab_duo) and a new governance page at /-/settings/gitlab_duo/governance, as described in the linked issue.

The implementation mirrors the existing Configuration card pattern.

The new governance settings page is blank except for the breadcrumbs.

References

Related to https://gitlab.com/gitlab-org/gitlab/-/work_items/597747

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the feature flag in rails console:

   Feature.enable(:gitlab_duo_governance_settings)

2. Visit a top-level group's Duo settings page: http://127.0.0.1:3000/groups/<group>/-/settings/gitlab_duo
3. Verify the Governance card is rendered
4. Click Change governance — verify it navigates to /-/settings/gitlab_duo/governance
5. Verify the governance page renders without errors and shows correct breadcrumbs
6. Disable the feature flag and verify the card and page are no longer accessible

MR acceptance checklist

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