Add warning banner for old versions and dev version

[jni]

Google returns /dev/ docs, causing confusion for users, e.g. scikit-image/scikit-image#4092. We should have a banner in the dev docs warning that they might include unreleased features.

We could even get fancy and only show this when we detect a difference between the current page and the /stable one.

[sciunto]

I'm not an expert in page ranking. Would it be possible to set a robot.txt on the server that would prevent search engines robots for crawling these pages?

[jni]

Yes, I think we already do this for the older versions.

[alexdesiqueira]

I guess a warning like "that's the documentation for the development version" and a link for the stable release should be sufficient in this case @jni. If we implement the "fancy" way, people with stable could not recognize there are distinct versions around.

[jni]

@alexdesiqueira note that I still want to keep the sidebar with all the versions. My proposal is just about whether to show a big red warning banner as well.

[alexdesiqueira]

We could even get fancy and only show this when we detect a difference between the current page and the /stable one.

@jni I said that referring to this specifically.

[hmaarrfk]

Users don't really understand what "développer" versions. Keep it simple guys. A global banner and a robots file will go a really long way

[soupault]

Thanks to @jni, I decided to investigate scikit-learn website. Here is what they do - https://scikit-learn.org/versionwarning.js