Document best practises on working with multiple environments

[abuehrle]

We should create a topic (and maybe a blog) on the best practises on using Flux in multiple environments: Test, staging, and production.

[stefanprodan]

We've published two examples on how to structure repositories for Flux v2:

• https://github.com/fluxcd/flux2-kustomize-helm-example
• https://github.com/fluxcd/flux2-multi-tenancy

Closing this issue as Flux v1 is in maintenance mode.