Add GitHub workflow for docs deployments

[danielledeleo]

Proposed changes

The NGINX docs site is migrating to self hosting. This PR adds support for a workflow to build, push, and provide PR previews. See: https://github.com/nginxinc/docs-actions

Checklist

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

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

[mjang]

Oh, I think I "get it," you've taken the "caller-example" template from https://github.com/nginxinc/docs-actions