Add documentation: Vscode jsonnet plugin

[Vincere1st]

Add in documentation how use the jsonnet plugin in vscode.

[jeremyVignelles]

That doc does exist in our internal azure repo. We just need to make it open-source (and translate it ?).

EDIT: The steps are roughly these:

• If ddb is not installed locally, open a remote vscode on the devbox (remote SSH for virtualbox, remote WSL for WSL2)
• Install the grafana jsonnet plugin: https://github.com/grafana/vscode-jsonnet
• configure the lib folder to point to the path where the ddb.libjsonnet file is stored

[Vincere1st]

Yes add it to the Doc, please on the github.io