When the preview feature will be available

[sunny0826]

I've observed that the preview feature is already active on the jekyll-preview.yml workflow at https://github.com/github/opensource.guide/blob/main/.github/workflows/jekyll-preview.yml. However, the README documentation still indicates that this feature is not publicly available. Given that I've seen some third-party Actions have implemented this capability, I'm inquiring about the timeline for when this feature will be officially rolled out for general use.