Document JSON formats

Good to document these formats separately from commands that happen to
use them.

Eventually I would like this and `builtins.derivation` to refer to a
store section on derivations that is authoritative, but that doesn't yet
exist, and will take some time to make. So I think we're just best off
merging this now as is.

Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

doc/manual/src/json/store-object-info.md

@@ -0,0 +1,97 @@
+# Store object info JSON format
+
+> **Warning**
+>
+> This JSON format is currently
+> [**experimental**](@docroot@/contributing/experimental-features.md#xp-feature-nix-command)
+> and subject to change.
+
+Info about a [store object].
+
+* `path`:
+
+  [Store path][store path] to the given store object.
+
+* `narHash`:
+
+  Hash of the [file system object] part of the store object when serialized as a [Nix Archive](#gloss-nar).
+
+* `narSize`:
+
+  Size of the [file system object] part of the store object when serialized as a [Nix Archive](#gloss-nar).
+
+* `references`:
+
+  An array of [store paths][store path], possibly including this one.
+
+* `ca` (optional):
+
+  Content address of this store object's file system object, used to compute its store path.
+
+[store path]: @docroot@/glossary.md#gloss-store-path
+[file system object]: @docroot@/store/file-system-object.md
+
+## Impure fields
+
+These are not intrinsic properties of the store object.
+In other words, the same store object residing in different store could have different values for these properties.
+
+* `deriver` (optional):
+
+  The path to the [derivation] from which this store object is produced.
+
+  [derivation]: @docroot@/glossary.md#gloss-store-derivation
+
+* `registrationTime` (optional):
+
+  When this derivation was added to the store.
+
+* `ultimate` (optional):
+
+  Whether this store object is trusted because we built it ourselves, rather than substituted a build product from elsewhere.
+
+* `signatures` (optional):
+
+  Signatures claiming that this store object is what it claims to be.
+  Not relevant for [content-addressed] store objects,
+  but useful for [input-addressed] store objects.
+
+  [content-addressed]: @docroot@/glossary.md#gloss-content-addressed-store-object
+  [input-addressed]: @docroot@/glossary.md#gloss-input-addressed-store-object
+
+### `.narinfo` extra fields
+
+This meta data is specific to the "binary cache" family of Nix store types.
+This information is not intrinsic to the store object, but about how it is stored.
+
+* `url`:
+
+  Where to download a compressed archive of the file system objects of this store object.
+
+* `compression`:
+
+  The compression format that the archive is in.
+
+* `fileHash`:
+
+  A digest for the compressed archive itself, as opposed to the data contained within.
+
+* `fileSize`:
+
+  The size of the compressed archive itself.
+
+## Computed closure fields
+
+These fields are not stored at all, but computed by traverising the other other fields across all the store objects in a [closure].
+
+* `closureSize`:
+
+  The total size of the compressed archive itself for this object, and the compressed archive of every object in this object's [closure].
+
+### `.narinfo` extra fields
+
+* `closureSize`:
+
+  The total size of this store object and every other object in its [closure].
+
+[closure]: @docroot@/glossary.md#gloss-closure