Add a chapter showing and example how to plot a simple function with
matplotlib. This commit add eps-files to the artifacts of the CI.

Use CI_REGISTRY_IMAGE instead of a combination of CI_REGISTRY and
CI_PROJECT_PATH_SLUG to fix registry image path.

Use CI_PROJECT_PATH_SLUG instead of CI_PROJECT_PATH in docker image path, since
the registry path must be lowercase.

Fix the gitlab-ci configuration by adding quotes to the artifacts path with
wildcards.

Add the gitignore file. Temporary vim file should be ignored, as well as
python script generated by doxec.

Add a CI pipeline with a single doxec task. The created python code examples are
packaged and can be downloaded on success.

This commit adds a chapter about the installation of python3, numpy, scipy and
matplotlib and a chapter with a first 'Hello World'-like example.