Show "Getting started" page after installation of extension

[BigMorty]

This getting started page will help users understand what features/functionality the extension has and should offer next steps and/or shortcuts to features. This should work similar to what the Python extension has done.

[bwateratmsft]

This might depend on microsoft/vscode#83451.

[dbreshears]

Python team is creating a getting started page experiment, will wait to see the outcome