[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

.gitlab-ci.yml

@@ -147,13 +147,13 @@ pages:
   stage: publish_doc
   script:
     - poetry install
-    - make docs
-    - mv docs/_build/html public
-    - ls public
+    - poetry run sphinx-apidoc -o docs .
+    - poetry run sphinx-build -b html docs public
   artifacts:
     untracked: true
     paths:
       - public
+    expire_in: 2 days
   rules:
     - if: $CI_COMMIT_TAG
       when: manual