Add documentation on public datasets

docs/public_dataset_browser/index.md

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+# Public Dataset Browser
+
+The "Public Dataset Browser" is a feature of
+[ChRIS_ui](https://github.com/FNNDSC/ChRIS_ui) which uses
+[NiiVue](https://github.com/niivue/niivue) to visualize neuroimaging data found in public feeds.
+
+https://app.fetalmri.org/niivue
+
+To add a public dataset, the feed must be public, and the plugin instance's output files must contain a
+file named `.is.chris.publicdataset`. The contents of this file can be arbitrary non-empty data.