Create reusable SidebarBlock component

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The following discussion from !56822 (merged) should be addressed:

• @markrian started a discussion: (+1 comment)

  nitpick: We should use GitLab UI's utilities:

        class="gl-display-block"

There is frequent confusion about this sidebar-specific .block class. Let's create a SidebarBlock component that can be re-used, so we can avoid this confusion in future work, and provide a SSoT if/when we remove this .block class in favor of gitlab ui utility classes.