Warn users clearly when viewing older documentation

[cknight]

The API manuals let you choose which version of the library/module you want to see, with the latest version marked 'latest' and if you choose one which is not the latest you get a button 'Go to Latest'. However, this doesn't seem clear enough you are visiting from a search engine for example which gives you an older version.

A better way would be to have a banner which clearly denotes you aren't looking at the latest docs. Here's what the Preact docs look like if you view an older version for example:

[Hajime-san]

Agree this. Google search result often picks older information.