Improving the DX of manually configuring preview.

[jacobcarrington]

Recently I've been having issues which Gatsby Cloud and it's inability to automatically configure the preview service in DatoCMS which leaves me in the position of having to install manually. Some points on confusion for me in this process were:

1. Auth token Field - It's described as a "The authentication token for a private Gatsby Cloud instance". I'm hunting around GC looking for a token where as it is a DatoCMS API token. Just call it that, and avoid any confusion.
2. DatoCMS API token configuration. Document which token is required. Is it read-only? Which APIs does it need access to. Document the exact configuration to make it work.