docs: Add (not installed) man pages for composefs and zstd:chunked

These aren't installed yet so they'll just live in the git repository
but hopefully we can expand them and use them as a central reference
point for "how do I do (zstd:chunked)|(composefs)". We have copies
of that kind of instruction in other downstream places but
I think here makes the most sense.

Signed-off-by: Colin Walters <walters@verbum.org>

docs/containers-storage-composefs.md

@@ -0,0 +1,38 @@
+## containers-storage 1 "August 2024"
+
+## NAME
+containers-storage-composefs - Information about composefs and containers/storage
+
+
+## DESCRIPTION
+
+To enable composefs at a baseline requires the following configuration in `containers-storage.conf`:
+
+```
+[storage.options.overlay]
+use_composefs = "true"
+```
+
+However at the current time, composefs requires zstd:chunked images, so first
+you must be sure that zstd:chunked is enabled. For more, see [zstd:chunked](containers-storage-zstd-chunked.md).
+
+Additionally, not many images are in zstd:chunked format. In order to bridge this gap,
+`convert_images = "true"` can be specified which does a dynamic conversion; this adds
+latency to image pulls.
+
+Putting these things together, the following is required (in addition to the above config).
+
+```
+pull_options = {enable_partial_images = "true", use_hard_links = "false", ostree_repos="", convert_images = "true"}
+```
+
+## BUGS
+
+- https://github.com/containers/storage/issues?q=is%3Aissue+is%3Aopen+label%3Aarea%2Fcomposefs
+
+## FOOTNOTES
+The Containers Storage project is committed to inclusivity, a core value of open source.
+The `master` and `slave` mount propagation terminology is used in this repository.
+This language is problematic and divisive, and should be changed.
+However, these terms are currently used within the Linux kernel and must be used as-is at this time.
+When the kernel maintainers rectify this usage, Containers Storage will follow suit immediately.

docs/containers-storage-zstd-chunked.md

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+## containers-storage 1 "August 2024"
+
+## NAME
+containers-storage-zstd-chunked - Information about zstd:chunked
+
+
+## DESCRIPTION
+
+The traditional format for container image layers is [application/vnd.oci.image.layer.v1.tar+gzip](https://github.com/opencontainers/image-spec/blob/main/layer.md#gzip-media-types).
+More recently, the standard was augmented with zstd: [application/vnd.oci.image.layer.v1.tar+zstd](https://github.com/opencontainers/image-spec/blob/main/layer.md#zstd-media-types)
+which is a more modern and efficient compression format.
+
+`zstd:chunked` is a variant of the `application/vnd.oci.image.layer.v1.tar+zstd` media type that
+uses zstd [skippable frames](https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md#skippable-frames)
+to include additional metadata (especially a "table of contents") that includes the SHA-256 and offsets of individual chunks of files.
+Additionally chunks are compressed separately. This allows a client to dynamically fetch only content which
+it doesn't already have using HTTP range requests.
+
+To enable zstd:chunked requires the following changes to `containers-storage.conf`:
+
+```
+pull_options = {enable_partial_images = "true", use_hard_links = "false", ostree_repos=""}
+```
+
+This option may or may not be enabled by default in your distribution/operating system.
+
+## INTERNALS
+
+At the current time the format is not officially standardized or documented beyond
+the comments and code in the reference implementation. At the current time
+the file with the most information is [pkg/chunked/internal/compression.go](https://github.com/containers/storage/blob/39d469c34c96db67062e25954bc9d18f2bf6dae3/pkg/chunked/internal/compression.go).
+The above is a permanent link for stability, but be sure to check to see if there are newer changes too.
+
+## BUGS
+
+- https://github.com/containers/storage/issues?q=is%3Aissue+label%3Aarea%2Fzstd%3Achunked+is%3Aopen
+
+## FOOTNOTES
+The Containers Storage project is committed to inclusivity, a core value of open source.
+The `master` and `slave` mount propagation terminology is used in this repository.
+This language is problematic and divisive, and should be changed.
+However, these terms are currently used within the Linux kernel and must be used as-is at this time.
+When the kernel maintainers rectify this usage, Containers Storage will follow suit immediately.