[infra] Consider displaying future pages like blog post and announcements, at least conditionally

[chalin]

I had turned off the Hugo generation of future posts (controlled by the -F, --buildFuture flag) because I wanted PR previews to show banners as they would appear in production. But, while attempting to review #4996, I see how useful it can be to support the preview of pages scheduled for publication at a future date.

There are a number of ways to approach this. Here are some thoughts:

• Always generate future pages. Con: preview doesn't reflect that the production server will look like once the PR is merged.
• Always generate future pages, but have them conditionally displayed in one's browser (using a CSS class that we use to toggle the page or announcement display)
• Render the build commands sensitive to a marker (a bit like the /fix:* commands), that would allow us to control (either through the PR title, or comment, or something similarly easy to update while viewing the PR) whether Hugo builds with --buildFuture. Note that we don't build via a GH workflow; it's Netlify that builds and deploys previews, so what ever trigger mechanism we adopt, the Netlify build process needs to be able to have easy access to the trigger source (be it PR title, etc).

Thoughts?