Upload generated documentation to our webserver

This merge requests adds a create_docs task to our .gitlab-ci.yml. The task will generate docs using sphinx and then push the results to our web server using rsync.

The job uses 3 variables, but only two of them are exposed in the CI configuration:

create_docs:
(...)
  variables:
    docs_upload_host: hosted-024-173.rz.uni-augsburg.de # hostname of target web server
    docs_path_component: icartt # if your path/URL should contain a stub, different than the (Gitlab) project name.

The third variable is docs_ssh_key which is private an thus has to be configured as a masked variable using the Gitlab Web UI.

The proposed task fetches the current projects version using poetry version -s, e.g. for the current HEAD it would upload to: https://mbees.med.uni-augsburg.de/docs/icartt/2.0.0-rc1/

You may find some internal documentation on how to set it up on https://mbees.med.uni-augsburg.de/dokuwiki/doku.php?id=it:services:documentationhosting