i.e. https://code.visualstudio.com/api/extension-guides/tree-view#tree-view -- the docs don't say it works with the References view container, but it does.

  "contributes": {
    "views": {
      "references-view": [
        {
          "id": "commentReferences",
          "name": "Comment References"
        },
        ...
      ]
    },
   ...

Also need to register and implement the tree data provider, e.g. vscode.window.registerTreeDataProvider('commentReferences', new CommentReferencesProvider());.