[Hubs] Document how to export multiple scopes

[flanakin]

⚠️ Problem

It's not completely clear to people that they can export multiple billing accounts and subscriptions or that they can extend the platform to ingest data from other clouds.

🛠️ Solution

Update the documentation to explicitly talk about what's possible when configuring scopes/exports.

🙋‍♀️ Ask for the community

We could use your help:

1. Please vote this issue up (👍) to prioritize it.
2. Leave comments to help us solidify the vision.