Warn when viewing outdated docs

[PBfordev]

When googling for wxWidgets class to see its docs, usually the top result is for v3.0 or something, i.e., severally outdated.

I know nothing about webservers but I wonder if there is not some easily done and not performance taxing magic (htaccess etc.) that could inject a warning to html docs that are not for the master or the stable version, i.e., something like boost or CMake docs do.