pkgdown action updates

[aylapear]

• trying out a new dev version of pkgdown action so that it builds the pkgdown site on the dev branch (but does not push to the gh-pages branch/update our actual website). This allows us to check that it builds properly before we accept and it gets pushed. Current workflow the pkgdown website isn't built until we accept a pull request so we can't check if its broken until its too late.
• adding private repo secrets to fix dependency issue