Generate Sphinx doc automatically on a separate repository

This MR proposes some changes for handling the Sphinx documentation:

• A CI job build-doc to generate Sphinx HTML documentation and export it as an artifact.
• A CI job release-doc to push the HTML documentation to a dedicated repository
• Instead of having the autogenerated files (in sphinxdocs/build and sphinxdocs/apidocs) in the FACILE-RS repository, the repository containing the HTML documentation is included as a submodule.

This would clearly establish which files are necessary to generate the documentation and which one are automatically generated. It would also avoid to have autogenerated files in the repository and to clutter to repo's history with changes in these files.