Document rootfs in overlays and images

- Closes: #1487

Signed-off-by: Jonathon Anderson <janderson@ciq.com>

userdocs/contents/images.rst

@@ -36,6 +36,47 @@ Apptainer is the most utilized image management tool. You can use
 any of these to create and manage the containers to be later imported
 into Warewulf.
 
+Structure
+=========
+
+A Warewulf container is a directory that populates the runtime root file system of a cluster
+node. The container source directory must contain a single ``rootfs`` directory which represents the
+actual root directory for the overlay.
+
+.. code-block:: none
+
+  /var/lib/warewulf/chroots/rockylinux-9
+  └── rootfs
+      ├── afs
+      ├── bin -> usr/bin
+      ├── boot
+      ├── dev
+      ├── etc
+      ├── home
+      ├── lib -> usr/lib
+      ├── lib64 -> usr/lib64
+      ├── media
+      ├── mnt
+      ├── opt
+      ├── proc
+      ├── root
+      ├── run
+      ├── sbin -> usr/sbin
+      ├── srv
+      ├── sys
+      ├── tmp
+      ├── usr
+      └── var
+
+Warewulf containers are built (e.g., with ``wwctl container build``) into compressed images for
+distribution to cluster nodes.
+
+.. code-block:: none
+
+  /var/lib/warewulf/provision/container
+  ├── rockylinux-9.img
+  └── rockylinux-9.img.gz
+
 Importing Images
 ================
 

userdocs/contents/overlays.rst

@@ -14,6 +14,20 @@ Warewulf addresses cluster node configuration with its **overlay** system. Overl
 of files and templates that are rendered and built per-node and then applied over the image
 image during the provisioning process.
 
+Structure
+=========
+
+An overlay is a directory that is applied to the root of a cluster node's runtime file system. The
+overlay source directory should contain a single ``rootfs`` directory which represents the actual
+root directory for the overlay.
+
+.. code-block:: none
+
+  /usr/share/warewulf/overlays/issue
+  └── rootfs
+      └── etc
+          └── issue.ww
+
 System and runtime overlays
 ===========================
 
@@ -24,6 +38,18 @@ A node or profile can configure an overlay in two different ways:
 * An overlay can be configured to also apply periodically while the system is running. These overlays
   are called **runtime overlays**.
 
+Overlays are built (e.g., with ``wwctl overly build``) into compressed overlay images for
+distribution to cluster nodes. These images typically match these two use cases: system and
+runtime. As such, each cluster node typically has two overlay images.
+
+.. code-block:: none
+
+  /var/lib/warewulf/provision/overlays/tn1
+  ├── __RUNTIME__.img
+  ├── __RUNTIME__.img.gz
+  ├── __SYSTEM__.img
+  └── __SYSTEM__.img.gz
+
 Distribution and site overlays
 ==============================