Add linked resource widget on the UI

What does this MR do and why?

Related to #230852 (closed)

This adds a nonfunctional UI widget for Linked resources. This feature is behind a feature flag and not enabled anywhere. The button click is non-functional.

I am breaking down the UI changes in simple mergeable MRs.

Screenshots or screen recordings

How to set up and validate locally

1. Enable the feature flag incident_resource_links_widget
2. Open any incident detail page and check the widget.
3. You need at least reporter permissions and GitLab Premium

MR acceptance checklist

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

• I have evaluated the MR acceptance checklist for this MR.

Related to #230852 (closed)