[REF] Combine Stardog and graphDB setup instructions (#92)

* Explain the difference between backend and graph

* Explain license differences

* Explain changes to env variables

* Describe initial security setup

* Adding DB user

* Split upload instructions for graph backend

Fixes #93

* Use consistent user name templates across both backends

Fixes neurobagel/api#119

* Also talk about query tool

and show it with a nicer figure