[ci] #165: Simplify the commands to build the doc

Build directly into public folder without cd
Remove make usage, which can be uninstalled from the images
`make` was used from the Makefile in 'duniterpy' folder
and from Sphinx doc generation into `docs' folder

Keep Sphinx installed in the job
Was installed in the image to speed-up the job,
but that’s a job run not that often, so, that’s fine
It’s better to keep control over Sphinx version from pyproject.toml

Add artifact expiration