Separate prod and dev sites; update actions for node v24

[ahocevar]

This pull request changes the github actions to produce gh-pages websites using a gh-pages branch, instead of the GitHub Pages action.

To make this work, the repository configuration for Pages needs to be changed to use a branch, and the branch's name is "gh-pages", and the directory is "/ (root)". In order to be able to make this change in the GitHub settings, the gh-pages branch must exist. To create one, something like this will do:

git checkout --orphan gh-pages
git reset --hard
git commit --allow-empty -m "Initial gh-pages"
git push origin gh-pages

When configured, a push to main will trigger a publish to /dev/, and a v*.. tag push will trigger a publish to / (production)

[m-mohr]

I updated the settings, the branch already exists.