[opct] add assets cmd docs to development guide

There may be some situations where the plugin manifests will need to be modified to workaround unforeseen issues. We should document how to use the assets command, edit plugin manifests, and use them.

Make sure its clear this wouldn't be viable for certification and would only be used for troubleshooting or bug fixes.

Reference: https://github.com/redhat-openshift-ecosystem/provider-certification-tool/pull/6#discussion_r900503537