Publish build html pages automatically

This will avoid the confusion as of e.g. yesterday how docs show up on developer.puri.sm.

Needs the protected variables within Gitlab's UI

• RSYNC_TARGET: remote host and folder
• SSH_USER: ssh user to log in as
• SSH_PORT: sshd port on the remote side
• SSH_KEY: ssh private key
• SSH_HOST_KEY: ssh host key of target gathered via e.g. ssh-keyscan

filled in to work.

A version churning in my personal repo can be seen in action here:

https://source.puri.sm/guido.gunther/developer.puri.sm/pipelines/2883

published here

https://storage.puri.sm/doctest/_build/html/

@david.boddie I can help with filling these before merging this MR.

Closes: #14 (closed)