Adding interactivity to the Getting Started page

[mradcliffe]

Is your feature request related to a problem? Please describe

Follow-up from #27

Instead of telling of users how to find something with text we should use links or buttons to open or focus the relevant panel of VS Code.

Describe the solution you'd like

It should be possible to use a messaging pattern to communicate between the extension's panel html and the extension itself.

There's a postMessage method available on window.parent in a WebView DOM, and then the extension can listen for those messages. And vice versa.

The extension can then invoke other parts of VS Code API to open different parts of the site.

Describe alternatives you've considered

Additional context