in various docs, we reference setting environment variables, but we don’t have a single, dedicated doc that breaks down how and where to set environment variables depending on deployment method. this makes it hard for users to understand where to start and what’s appropriate for their setup.

• new doc at docs/admin/setup/env-vars
• include
  • what are environmental variables
  • when to use them vs
  • deployment-specific options (+link out)
    • k8s, coder.env, docker-compose, other?
  • example options (+link to /cli/server)