Mention that each node is expected to correspond to one graph database

[surchs]

We should also explicitly distinguish "public" from "local" nodes (i.e., the former could also be nodes deployed locally at an institute or lab, but they are data that can be openly queried). This could potentially live on its own page ("About Neurobagel graph databases" or "About Neurobagel nodes"?)

Originally posted by @surchs in #133 (comment)

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