Add docs-build-push github workflow

[nginx-jack]

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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.

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

NONE