Get IaC production-ready: documentation

[bsweger]

If we decide to proceed with Pulumi as a framework for managing hubverse cloud infrastructure, we need to ensure that the process is well-documented. We're assuming that the target audience(s) are people who develop Hubverse tooling (as opposed to Hubverse admins and users).

Definition of done:

• [x] Documentation exists in the repo (e.g., README or similar)
• [x] Documentation has high-level instructions for the use case of onboarding a hub to Hubverse-owned AWS resources (basically, update the YAML config)
• [x] Documentation has more detailed instructions for those who need to update the Pulumi code itself (for example, to fix a bug or add a new AWS resource to support additional features)

[bsweger]

There is currently an open PR that address two of the three items on this ticket: https://github.com/Infectious-Disease-Modeling-Hubs/hubverse-infrastructure/pull/44

This PR from last week addresses the third item: https://github.com/Infectious-Disease-Modeling-Hubs/hubverse-infrastructure/pull/42 (and Matt confirmed he was able to get the product set up)

[bsweger]

Cleaning up--never closed this after the PRs were merged!