Preesm Snapshot repo and DFTools archived

We have setup an update site for hosting the last 10 snapshot versions of Preesm at https://preesm.github.io/preesm-snapshot-site/.

The procedure is similar to the one presented in our documentation.

Also, following the release 3.0.0, the DFTools repository has been archived. That means the repository is now read-only and we stop its support. All the releases are still available on our archived repository at http://preesm.insa-rennes.fr/repo/update-site-backups/.

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