Rename doc/manual{src -> source}

This is needed to avoid this
https://github.com/mesonbuild/meson/issues/13774 when we go back to
making our subproject directory `src`.

doc/manual/source/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](./store-path.md) 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.