Write a simple "quick start" for swh-graph

Write a quick start guide that document how to have a running swh-graph api server running from start to finish, using the compressed graph available on git-annex. The kind of doc that a user can copy/paste commands in a terminal and "it works"™

Can be based on docker (if so, don't forget to document "docker volume" stuff where the actual data are stored).

Migrated from T2530 (view on Phabricator)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information