docs: simplify GCP quickstart docs

We now have Coder VM listed on [Google Cloud Marketplace](https://console.cloud.google.com/marketplace/product/coder-enterprise-market-public/coder-v2)

We should simply our [GCP docs](https://github.com/coder/coder/blob/main/docs/platforms/google-cloud-platform.md) similar to [AWS marketplace docs](https://github.com/coder/coder/blob/main/docs/platforms/aws.md).