Add examples for viewsContainers

Per: https://github.com/microsoft/vscode-discussions/discussions/2981

There should also be examples for activity bar, panel, and secondarysidebar, since the viewsContainers API is not detailed here, it was hard for me to discover.

https://github.com/microsoft/vscode-extension-samples/blob/0c01b9ad1cbd411b1baf44dc87d040c6258c6440/webview-view-sample/package.json#L24