Iterate on Flux documentation

Now that we have a general Flux doc and a quickstart tutorial page, we should consider how to make the documentation better for production users.

Proposed outline of the Flux doc

General introduction to Flux
Repository configuration
- Provide a recommended repo structure, and a diagram like the one we have on the GitOps workflow page
Bootstrap installation
- Quickstart
  - Link to the tutorial
- Infrastructure as code
  - Instructions to use Flux with Terraform and a deploy token
Access keys
- What keys does Flux need to do thing?
  - Access the repo
  - Access the container registry from a service account
  - GitLab → Flux notification
  - Flux → GitLab notification