document store objects in terms of their constituent parts

this also rephrases the introductory sentence to be more general, in order to
avoid the same word being repeated in short succession.

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

@@ -0,0 +1,10 @@
+## Store Object
+
+A Nix store is a collection of *store objects* with *references* between them.
+A store object consists of
+
+  - A [file system object](./file-system-object.md) as data
+  - A set of [store paths](@docroot@/glossary.md#gloss-store-path) as references to other store objects
+
+Store objects are [immutable](https://en.wikipedia.org/wiki/Immutable_object):
+Once created, they do not change until they are deleted.