Document password protection for GitHub Actions

[gadkins]

Add a description of how to use password protected environments with GitHub Actions

Describe this in detail with screenshots:

https://github.com/UffizziCloud/example-voting-app/blob/8f78f9204c8869aca538cb929d49c5b1074da8ff/.github/workflows/uffizzi-previews.yml#L179-L180

Add it to this article https://docs.uffizzi.com/guides/password-protected/

[axisofentropy]

https://github.com/UffizziCloud/preview-action#url-username-and-url-password