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/background.md

@@ -0,0 +1,81 @@
+# Kata Containers architecture background knowledge
+
+The following sections explain some of the background concepts
+required to understand the [architecture document](README.md).
+
+## Root filesystem
+
+This document uses the term _rootfs_ to refer to a root filesystem
+which is mounted as the top-level directory ("`/`") and often referred
+to as _slash_.
+
+It is important to understand this term since the overall system uses
+multiple different rootfs's (as explained in the
+[Environments](README.md#environments) section.
+
+## Container image
+
+In the [example command](example-command.md) the user has specified the
+type of container they wish to run via the container image name:
+`ubuntu`. This image name corresponds to a _container image_ that can
+be used to create a container with an Ubuntu Linux environment. Hence,
+in our [example](example-command.md), the `sh(1)` command will be run
+inside a container which has an Ubuntu rootfs.
+
+> **Note:**
+>
+> The term _container image_ is confusing since the image in question
+> is **not** a container: it is simply a set of files (_an image_)
+> that can be used to _create_ a container. The term _container
+> template_ would be more accurate but the term _container image_ is
+> commonly used so this document uses the standard term.
+
+For the purposes of this document, the most important part of the
+[example command line](example-command.md) is the container image the
+user has requested. Normally, the container manager will _pull_
+(download) a container image from a remote site and store a copy
+locally. This local container image is used by the container manager
+to create an [OCI bundle](#oci-bundle) which will form the environment
+the container will run in. After creating the OCI bundle, the
+container manager launches a [runtime](README.md#runtime) which will create the
+container using the provided OCI bundle.
+
+## OCI bundle
+
+To understand what follows, it is important to know at a high level
+how an OCI ([Open Containers Initiative](https://opencontainers.org)) compatible container is created.
+
+An OCI compatible container is created by taking a
+[container image](#container-image) and converting the embedded rootfs
+into an
+[OCI rootfs bundle](https://github.com/opencontainers/runtime-spec/blob/main/bundle.md),
+or more simply, an _OCI bundle_.
+
+An OCI bundle is a `tar(1)` archive normally created by a container
+manager which is passed to an OCI [runtime](README.md#runtime) which converts
+it into a full container rootfs. The bundle contains two assets:
+
+- A container image [rootfs](#root-filesystem)
+
+  This is simply a directory of files that will be used to represent
+  the rootfs for the container.
+
+  For the [example command](example-command.md), the directory will
+  contain the files necessary to create a minimal Ubuntu root
+  filesystem.
+
+- An [OCI configuration file](https://github.com/opencontainers/runtime-spec/blob/main/config.md)
+
+  This is a JSON file called `config.json`.
+
+  The container manager will create this file so that:
+
+  - The `root.path` value is set to the full path of the specified
+    container rootfs.
+
+    In [the example](example-command.md) this value will be `ubuntu`.
+
+  - The `process.args` array specifies the list of commands the user
+    wishes to run. This is known as the [workload](README.md#workload).
+
+    In [the example](example-command.md) the workload is `sh(1)`.