Docs on appending the user agent with relevant atlantis information

[nitrocode]

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• I'd be willing to implement this feature (contributing guide)

Describe the user story

I'd like to differentiate between atlantis applies for infra changes vs other CI changes vs local changes

Describe the solution you'd like

I'd like a document explaining how to set a custom env var to provide relevant and useful atlantis context per PR within the custom user agent so in cloudtrail we can differentiate the changes

https://developer.hashicorp.com/terraform/language/settings/backends/s3#configuring-custom-user-agent-information

workflows:
  default:
    plan:
      steps:
        - env:
            name: TF_APPEND_USER_AGENT
            command: 'echo "Atlantis v0.20.1 'terraform$ATLANTIS_TERRAFORM_VERSION apply' by $USER_NAME in workspace $WORKSPACE, dir $REPO_REL_DIR, PR https://github.com/$BASE_REPO_OWNER/$BASE_REPO_NAME/pulls/$PULL_NUM"'

More context could be added

https://www.runatlantis.io/docs/custom-workflows.html#custom-run-command
https://www.runatlantis.io/docs/custom-workflows.html#environment-variable-env-command

Describe the drawbacks of your solution

None

Describe alternatives you've considered

None