github-pages-deploy-action is overwriting Documenter builds

[Roger-luo]

in latest version (v3.6) this github action make CLEAN: true becomes the default behaviour, which as a result will make PkgPage overwites all the previous build of Documenter.

should change it in the docs of this package to add CLEAN: false explicitly?

[tlienart]

thanks for this! yes that would be good can you PR?