storage API server: nicer landing page

Running a local storage server invites the (developer) user to visit the HTTP landing page:

 * Running on http://localhost:5002/ (Press CTRL+C to quit)

visiting that page works in the browser, which shows a bare bone "SWH Storage API server" message, but fails with a 404 werkzeug traceback on the console.

We should make the landing page nicer by, e.g., showing a browsable REST API documentation of the storage API server.

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Migrated from T1194 (view on Phabricator)