FSharp docs expose pre-release functionality

https://github.com/fsharp/fsharp-core-docs/blob/7d5b8049262e298e6e620ff308ada7999b9a2d81/.github/workflows/docs.yml#L29

My main question is: Is it intentional to have the docs show pre-release functionality?

I was writing a little app and forgot some syntax when using map. I checked the docs and noticed that it exposed a Keys property. I tried to use that instead of what I was doing. After a bit of poking to see if I was somehow missing a namespace I was helped by another engineer who said that Keys is a dotnet 6 feature. I then tried to answer the question "What of this can I use, and what can't I use yet?" which I was unable to do easily.

If this isn't intentional, I could see a number of ways to help:

- Build the docs not off of main, but off a release branch.
- Build off the main branch, but leave it to maintainers to mark functionality "pre-release" through an attribute and remove them when the functionality reaches the current dotnet version
- This would probably be a larger effort, but enhance the docs so they can be contextual based on the dotnet version similar [to how the Microsoft docs here work](https://docs.microsoft.com/en-us/dotnet/api/system.collections.ienumerable?view=net-5.0)