Improve extension telemetry documentation

The documentation for [adding telemetry to extensions](https://code.visualstudio.com/api/extension-guides/telemetry) is unclear and incomplete. 

Add more descriptive steps about how extension authors can:

* Capture telemetry for an extension (`TelemetryReporter` ?) 
* Store telemetry events (`@vscode/extension-telemetry` ?)

@isidorn FYI