Documentation: update and improve developer documentation

[schu]

It would be great to..

• Clean up the existing documentation:
  • Move non-authorization docs (database setup) and frontend development to a separate file, e.g. docs/development.md
  • Verify the docs work (e.g. https://github.com/kinvolk/nebraska/blob/master/docs/authorization.md#deploying-nebraska-for-testing-on-local-computer-noop-authentication doesn't work as-is since NEBRASKA_DB_URL is not set)
• Update https://github.com/kinvolk/nebraska/tree/master/updaters/examples/demo (make the used app and group UUIDs match what is in Nebraska by default and don't use sample data UUIDs that we don't use AFAICT) and make it work out of the box
• Explain in more detail what https://github.com/kinvolk/nebraska/tree/master/updaters/examples/demo is, what it can be used for and how it's done.
  • A more simple and well documented fake-instance utility would be nice
  • The current version requires -instance-id but doesn't check for it -> SQL error in server. Should go run updaters/examples/demo/demo.go -instance-id $(uuidgen) work?