Add a salsa-ci.yml file and document in it why it is disabled

Before this commit also the CI/CD configuration file field at https://salsa.debian.org/python-team/packages/graph-tool/-/settings/ci_cd#js-general-pipeline-settings was changed from recipes/debian.yml@salsa-ci-team/pipeline to debian/salsa-ci.yml. Done now.

Note that this MR has zero pipelines, which proves the yaml file contents is correct in disabling the pipeline.