Archive documentation should have a banner that say that this is not the lastest doc

[ogoffart]

Many users can get confused when looking old documentation, and not realize it is an outaded docs.
(example #5019)

We should inject a banner somehow in the HTML of these old documentation.

The banner would just be a warning that this is an old version, maybe with a link to the new version.
We could also have a combo-box to switch version like cmake does (example from cmake: https://cmake.org/cmake/help/v3.24/command/file.html ).

I recommend that we run a command line to add a <script> to every file within the right directory of the www-releases repo.
Example (untested): find . -name "*.html" -exec sed -i '/<\/head>/i <script src="..."></script>' {} \;

But for future version we can also add the script directly before.