Banner: warn users on latest they're looking at dev docs and link to stable

[mdeff]

I've had multiple issues opened by users reading docs from latest (I don't know how they arrived there, maybe through a web search) without realizing those were for the development version, not the version they installed with pip or conda.

Could we extend the version banner to warn anytime people are not viewing stable? I've seen the discussion in #3481, where it's argued that this is complicated in general. I agree. But I think latest being the developement version is quite common. I'd propose three options for the banner:

1. no banner (or do it yourself)
2. banner warns on old and redirects to stable (what we have now)
3. banner warns on old and latest and redirects to stable (would cover many use cases in my opinion)

Example message from PyTorch.

[stsewd]

You can do 1 with https://github.com/humitos/sphinx-version-warning.

banner warns on old and latest and redirects to stable (would cover many use cases in my opinion)

I think we didn't do that because every project wants to be able to customize the wording and where to put the message, so is hard to cover all of these cases.

[mdeff]

I think a standard default would help a lot. The documentation could then redirect to https://github.com/humitos/sphinx-version-warning (or alternatives) for customization.

[humitos]

@mdeff Hi! We already have a warning banner implemented inside RTD (you can enable it from your admin's project). Although, as @stsewd said, it didn't cover all the cases and people had different ideas of what it was the best.

I implemented sphinx-version-warning as an high customizable alternative so each owner can show the banner where they like and how they like.

I'm afraid that we are not going to change RTD internal's behavior, but I'm happy to read any suggestion that you may have about that extension in case you find it useful for your use case.

[rmarzocchi84]

I have tried the a warning banner implemented inside RTD, but it do not show nothing. There is some option in the conf.py to set?

[no-response]

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