Proposed Webview API - Documentation

[tsalinger]

Testing #44466

• The documentation is missing an example on how to receive messages in the webview like:
  window.addEventListener("message", receiveMessage, false);
• The dispose() docs refer to a 'onDispose' function. Did you mean 'onDidDispose'?

[mjbvz]

First cut of documentation is here: microsoft/vscode-docs@d489410

Note that this is more tutorial documentation, not reference documentation. The reference documentation is generated from the API itself