Add check for external contributor

[nginx-jack]

Proposed changes

Community PR preview builds will always fail because forks cannot get access to github secrets.
NIC have implemented this on their repository with this PR nginx/kubernetes-ingress#6123, but ideally this would be managed directly in docs-actions, to avoid code duplication across OSS repos.

Expected Behavior

Creating a PR from a fork will not run the docs-actions, they will be skipped
Creating a PR directly on the repo will continue to operate as it does currently

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md and CHANGELOG.md)

[mjang]

One request for clarification... I'm not a dev, but it's based on my reading of L67 of the docs-build-push.yml file.

[lamATnginx]

Looks good to me!