Add Doc for Monorepo Support

[benjaminsehl]

We already support this, but we don't make it easy to figure out how.

Our CLI supports a path argument: https://shopify.dev/docs/custom-storefronts/hydrogen/cli/commands#build

and Worker Actions support a path field: https://github.com/Shopify/hydrogen/blob/main/.github/workflows/deploy.yml#L53

Let's just write a quick doc on how you can have your app build from within a monorepo.

[tomferez]

I have tried everything to no avail in order to set up a monorepo with yarn workspaces.

I always stumble upon the following error:

Launching the hydrogen app from its folder works well though - it just does not get launched with yarn run dev (even with the --path flag).