[repo] Show a warning for stable versions that newer docs are available.

[andrewnicols]

[~sarjona]: Here's the alternative for #1217.

I 100% do not agree with doing this and I have no intention of landing this. It's a blight on the documentation and I feel that it serves no useful purpose but to add more junk on the top of the page and to push the useful part of the docs out of the immediate code window.

The pages you want to add this to are for fully supported versions of the codebase. I also do not agree that we should point people to docs for unstable versions of the codebase.

I won't land this.