Add documentation around Environment Variable configuration

[sdedovic]

What kind of change does this PR introduce?

Doc Update

What is the current behavior?

This works for me with the following (abridged) terraform configuration:

terraform {
  required_providers {
    supabase = {
      source  = "supabase/supabase"
      version = "~> 1.0"
    }
  }
}

Not sure why this behavior isn't documented by y'all, makes using the provider in automated places much easier.

[sdedovic]

No idea why the PR checks are failing this is simply a documentation update

[sdedovic]

Also maybe consider adding it to your README and the docs here too?