docs: add STACKIT_SERVICE_ACCOUNT_KEY and STACKIT_PRIVATE_KEY envs

[marceljk]

Description

relates to -

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Acceptance tests got implemented or updated (see e.g. here)
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)

Testing instructions

• Use the following tf-file and run $ terraform apply:
  • This should result in an error, with the message that no credentials were found

provider "stackit" {
}

resource "stackit_key_pair" "keypair" {
  name       = "test-key-pair"
  public_key = chomp(file("~/.ssh/<path-to-your-ssh-key>.pub"))
}

• Read you service account key file into the env STACKIT_SERVICE_ACCOUNT_KEY:
  • $ export STACKIT_SERVICE_ACCOUNT_KEY=$(cat ./<path-to-your-sa-key>.json)
• Rerun $ terraform apply
  • This time it should work
• Cleanup by running $ terraform destroy