docs: Add new "howto write a lister tutorial" with unified lister api

This adds a new tutorial which details how to currently write the new listers (both incremental or stateless). This proposes a python template file to start a new lister.

At last, this renames the previous tutorial into tutorial-2017.

Related to swh-scheduler#3073 (closed)

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Migrated from D5069 (view on Phabricator)