Add examples for viewsContainers

[JustinGrote]

Per: microsoft/vscode-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.

vscode-extension-samples/webview-view-sample/package.json

Line 24 in 0c01b9a

"contributes": {