chore(docs): discovery documentation

docs/public_discovery.md

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+# Public data discovery configuration
+
+New in Bento v17.
+
+Previously, the public data configuration given to Katsu was applied on all the metadata contained in the service.
+This configuration declares which fields can be queried publicly for discovery purposes, which charts to display
+and which censorship rules to apply on the results.
+
+Katsu can hold multiple projects/datasets that may use different fields, require specific charts or custom
+`extra_properties` schemas at the project level.
+Therefore, there is a need to tailor the discovery configuration at different levels.
+
+Bento v17 gives the ability to specify a scoped Discovery configuration at the following levels:
+-   Dataset
+    -   Optional at dataset creation
+    -   For scoped queries on public endpoints targeting a project and dataset:
+        -   Katsu will use the dataset's discovery configuration
+        -   If no configuration is found, fallsback on the parent project's discovery
+-   Project
+    -   Optional at project creation
+    -   For scoped queries on public endpoints targeting a project only:
+        -   Katsu will use the project's discovery configuration
+        -   If no configuration is found, fallsback on the node's config
+-   Node
+    -   Optional during Katsu deployment
+    -   Uses the legacy `lib/katsu/config.json` file mount
+    -   For non-scoped queries on public endpoints:
+        -   Katsu will use the node's discovery, if there is one.
+        -   If no node configuration is found, Katsu will respond with a 404 status.
+    -   For scoped queries on public endpoints:
+        -   Katsu will fallback on the node's discovery if the project and/or dataset in the scope don't have one
+        -   If no node configuration is found, Katsu will respond with a 404 status.
+
+In previous versions of Bento, the bento_public web application could only show the aggregated data of all projects and datasets.
+Now, bento_public users can select a project/dataset scope in order to only retrieve the data contained in it.
+
+Given that projects/datasets use different fields or may have custom extra properties, depending on the study, you can now
+declare the fields and charts of interest at the relevant level.