Add a Welcome page

[ulugbekna]

When the extension is first installed, we should be able to open a "Welcome page" with some extension documentation, so that users installing the extension feel at more ease getting familiar with certain features of the extension.

Code example from the Go extension: https://github.com/golang/vscode-go/blob/e6c2d3ce23ebc26bee10d573f0444b619ab44b2e/src/welcome.ts

[ulugbekna]

Vscode also added a “tour” like view which allows to guide the user to set up their environment step by step, eg install opam if not installed, set up sandbox, it can make beginner experience extra nice