Documentation: Warnings about protected keys

related to https://earth.bsc.es/gitlab/digital-twins/de_340/project_management/-/issues/216#note_191156

The supported key is PEM as stated in

https://autosubmit.readthedocs.io/en/v4.0.0b/installation.html?highlight=pem#how-to-install

• Remark that the PEM key is the only tested ( others may be valid but some no)
• Add instructions prior to autosubmit before run it with a protected key ( ssh-add )
• Add an related error code in Autosubmit