Document minimal product metadata in CONTRIBUTING.md

[isidorn]

On Friday I tried to develop the chat extension, and I hit this issue #254160

We should document in our https://github.com/microsoft/vscode-copilot-chat/blob/main/CONTRIBUTING.md how to setup the needed metadata to have this working (via product.overrides.json)

@bpasero I am not sure what is the minimal metadata that we should suggest users to set - but ideally this does not expose any internal stuff

fyi @rebornix

[bpasero]

@isidorn I see that this only applies when you also have VS Code out of sources, which seems like a niche way of running, probably only done by us in the team. It may confuse people that just want to run the extension, so I wonder if it should still be added to the docs.

[isidorn]

Good point. Let's see what @rebornix thinks.