Mark “Work In Progress” documentation pages with their corresponding GitHub PR

[cuihtlauac]

New or updated documentation pages are dealt as GitHub pull requests, that's the easier way to go (several reasons). However, documentation PRs are different from “pure” code PR:

• Expects feedback from a wider audience
• Should trigger more discussion
• Tend to have a long maturation time
• Build can't fail
• Do not need testing
• Should be workable online only (i.e. w/o a dev set up)

This suggests an updated/dedicated workflow.

• Automatic squash, merge and publish (in main, staging or pr instance (see An Instance for Each Pull Request #1098)? - TBD)
• Automatic message to discuss.ocaml.org and/or slack?
• Automatic marking of the online page with a link to the open PR: “This is a WIP page, your feed-back would be highly appreciated + GH pull link”
• On refresh/update possibility to switch between old and new page, preferably with a button rather than using the address bar

I believe a fair share of that can be achieved using GitHub actions.