How to authenticate to host documentation in GH

These are the steps to follow:

• You have to generate locally a special public/private key pair with documenter tools (e.g. user="gridap", repo="Gridap.jl") https://juliadocs.github.io/Documenter.jl/stable/man/hosting/#travis-ssh
• The public key goes in the deploy keys section of the repo (do not forget to give it write access): https://github.com/USER/REPO/settings/keys
• The private key goes to the secrets section of the repo (never echo $DOCUMENTER_KEY in a workflow): https://github.com/USER/REPO/settings/secrets/actions