Language model provider guide

[isidorn]

Ref API microsoft/vscode#250007

We need a guide for extensions that want to provide language models.
So the producer, for the consumer part of this guide https://code.visualstudio.com/api/extension-guides/language-model

This does not have to be long.

Nick - do you have cycles to drive this. I am happy to sync and discuss about the content. Though we can wait first for Logan to finalize the API.

fyi @lramos15

[ntrogh]

@isidorn Suggest waiting for the API to be final.

[isidorn]

Makes sense.