Document new support for adding generic container configs

[travier]

Add docs for #4701 in https://coreos.github.io/rpm-ostree/container/ (and man page?).

See:

• #4703
• ostreedev/ostree-rs-ext#567
• https://github.com/opencontainers/image-spec/blob/main/config.md

[lmilbaum]

Can this issue be prioritized? I'm looking into configuring an annotation and I believe container_config key can be of help.

[travier]

Example in https://github.com/travier/ostree-containers/blob/main/.github/workflows/images.yml#L53 & https://github.com/travier/ostree-containers/blob/main/40/config.json.

I'll make a docs PR.