Add description for file system objects (#8500)

While this is not actually a notion in the implementation, it is
explicitly described in the thesis and quite important for understanding
how the store works.

Co-authored-by: John Ericson <git@JohnEricson.me>
Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

doc/manual/src/architecture/file-system-object.md

@@ -0,0 +1,64 @@
+# File System Object
+
+Nix uses a simplified model of the file system, which consists of file system objects.
+Every file system object is one of the following:
+
+ - File
+
+   - A possibly empty sequence of bytes for contents
+   - A single boolean representing the [executable](https://en.m.wikipedia.org/wiki/File-system_permissions#Permissions) permission
+
+ - Directory
+
+   Mapping of names to child file system objects
+
+ - [Symbolic link](https://en.m.wikipedia.org/wiki/Symbolic_link)
+
+   An arbitrary string.
+   Nix does not assign any semantics to symbolic links.
+
+File system objects and their children form a tree.
+A bare file or symlink can be a root file system object.
+
+Nix does not encode any other file system notions such as [hard links](https://en.m.wikipedia.org/wiki/Hard_link), [permissions](https://en.m.wikipedia.org/wiki/File-system_permissions), timestamps, or other metadata.
+
+## Examples of file system objects
+
+A plain file:
+
+```
+50 B, executable: false
+```
+
+An executable file:
+
+```
+122 KB, executable: true
+```
+
+A symlink:
+
+```
+-> /usr/bin/sh
+```
+
+A directory with contents:
+
+```
+├── bin
+│   └── hello: 35 KB, executable: true
+└── share
+    ├── info
+    │   └── hello.info: 36 KB, executable: false
+    └── man
+        └── man1
+            └── hello.1.gz: 790 B, executable: false
+```
+
+A directory that contains a symlink and other directories:
+
+```
+├── bin -> share/go/bin
+├── nix-support/
+└── share/
+```