Fix automatic generation of website

[traversaro]

The website was last generated in August 2021 (see http://robotology.github.io/ycm/gh-pages/git-master/index.html) I guess manually. We should fix the generation of the website, ideally automatically.

[drdanz]

I confirm that this was done manually by me, you probably just need to create an action that runs tools/gen-gh-pages.sh and push the gh-pages branch