Add Organization overview menu to organization sidebar

What does this MR do and why?

This MR adds an "Organization overview" menu to the organization sidebar (lib/sidebars/organizations/). The menu contains a single "Dashboard" item that links to the organization-scoped admin dashboard via the organization-aware admin_root_path helper.

Previously, this menu was added to the instance admin sidebar as part of a unified admin area. This MR reworks that approach: the instance admin sidebar is kept distinct from the organization sidebar, so the menu now lives in the organization sidebar instead.

The menu renders when all of the following conditions are met:

• The user is signed in.
• The org_admin_area feature flag is enabled for the organization.
• The user has the access_organization_admin_area permission on the organization.

Related issue: #587370 (closed)

Screenshots

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability. Please review the acceptance checklist.

Testing

1. Visit https://gdk.test:3443/o/default/admin as an instance admin or organization admin.
2. Observe 404 not found.
3. Enable org_stage_experimental feature flag.
4. Visit https://gdk.test:3443/o/default/admin as an instance admin or organization admin.
5. Observe the sidebar is the Organization overview with Dashboard subitem.