rely on modern GitHub Pages deployment through GitHub Actions instead of pushing to a specific branch (gh-pages). A cleaner behavior, similar to GitLab Pages.
improve documentation workflow to build also on PR but not deploy
remove ghp-import dependency since it's not necessary anymore
In this PR :
After merging this one, remember to remove gh-pages branch and switch to beta GitHub Actions in https://github.com/QGIS-Contribution/QGIS-ResourceSharing/settings/pages