docs: Split background and example out of arch doc

Move the background and example command details out of the architecture
doc and into separate files.

Partially fixes: #3246.

Signed-off-by: James O. D. Hunt <james.o.hunt@intel.com>

docs/design/architecture/example-command.md

@@ -0,0 +1,30 @@
+# Example command
+
+The following containerd command creates a container. It is referred
+to throughout the architecture document to help explain various points:
+
+```bash
+$ sudo ctr run --runtime "io.containerd.kata.v2" --rm -t "quay.io/libpod/ubuntu:latest" foo sh
+```
+
+This command requests that containerd:
+
+- Create a container (`ctr run`).
+- Use the Kata [shimv2](README.md#shim-v2-architecture) runtime (`--runtime "io.containerd.kata.v2"`).
+- Delete the container when it [exits](README.md#workload-exit) (`--rm`).
+- Attach the container to the user's terminal (`-t`).
+- Use the Ubuntu Linux [container image](background.md#container-image)
+  to create the container [rootfs](background.md#root-filesystem) that will become
+  the [container environment](README.md#environments)
+  (`quay.io/libpod/ubuntu:latest`).
+- Create the container with the name "`foo`".
+- Run the `sh(1)` command in the Ubuntu rootfs based container
+  environment.
+
+  The command specified here is referred to as the [workload](README.md#workload).
+
+> **Note:**
+>
+> For the purposes of this document and to keep explanations
+> simpler, we assume the user is running this command in the
+> [host environment](README.md#environments).