Document output of oci_image more clearly

The output of an `oci_image` and `oci_image_rule` is not clearly documented.

The only reference to the output as of today (taken from [here](https://github.com/bazel-contrib/rules_oci/blob/main/docs/image.md#oci_image)) is

> Produces a target [name].digest, whose default output is a file containing the sha256 digest of the resulting image. This is similar to the same-named target created by rules_docker's container_image macro.

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The output of the rule seems to be a directory. When I inspect the directory, I don't see much content. I would have expected to single file with an image file I could run though `crane export ` so I can inspect the file system contents of the image.

Start with docs/image.md#oci_image and inspect the oci_image and oci_image_rule outputs described in the issue. Clarify what the output directory and digest target contain, and explain the supported way to inspect or export the image. Done when the documentation resolves the discrepancy between the documented digest output and the observed directory.

Written by the indexing model from the issue text.