Serve pkgdown from docs/ master instead of gh-pages branch

[mlr-org]

Serve pkgdown from docs/ master instead of gh-pages branch

[mllg]

Do we really want this? Do we then get one extra commit message per commit?

[pat-s]

Similar as we do it in mlr currently: Build it (to check if everything is ok) but don't deploy. Update changed docs directly in the PR and merge silently.

I recently had a long discussion with Bernd and Jakob before about the best approach. Currently, there is no option that has no drawback.

The problem is that pkgdown::build_site() updates all .png files and the seed is not respected. This then sets the files to "changed" in git and we get a new commit deployed. Once they have resolved this issue we can again auto-deploy.

Big advantage of master/docs to gh-pages: PR preview of the changed site which is a very important feature.

As long as the site is minimal, we can serve through the gh-pages branch.

[pat-s]

To have the pkgdown site as a subdomain mlr3.mlr-org.com, we need to have it in master/docs because the deployment to gh-pages deletes the CNAME file on every push because its doing an orphan = T push.