docs: dedicated doc for uploading of OCI images

[sed-i]

Please describe the question or issue you're facing with "Manage resources - Charmcraft 3.5.3 documentation".

Trying to upload an oci image, I encountered two issues which I didn't find reference of in the docs:

1. The skopeo method (DIGEST=$(skopeo inspect $IMAGE | jq -r '.Digest')) for ghcr returned Unknown OCI image reference.
2. With the docker method (IMAGE_ID=$(docker images -q $IMAGE)) the upload succeeded, but resource-revisions lists amd64 only under platform, even though the tag is multiarch.

It would be handy to have a dedicated doc for uploading OCI images.

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Reported from: https://documentation.ubuntu.com/charmcraft/stable/howto/manage-resources/

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Thank you for reporting your feedback to us!

The internal ticket has been created: https://warthogs.atlassian.net/browse/CHARMCRAFT-644.

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[lengau]

A good first version of this would be "How to upload a rock as a resource," perhaps along with an enhancement for making this specific use case easy. From there it may even be okay to set the workflow for more complex cases to be "use skopeo to download the OCI as an OCI archive, then upload that."

Unfortunately, uploading a multi-arch OCI image is a pretty complex thing, as most tooling assumes you only want the version for your architecture and you've got to tell it in a million places that you actually want the multi-arch version.